Bases are very handy for miniatures. They can provide

• an additional source of creativity (customizing the base)
• extra protection
• ballast
• better gripping and transport of the miniature

All of my bases are designed and built to be used with magnets for storage and transport. First, here a couple of reasons why I bother:

Steel-Bottom Bases

Magnets are not very useful unless you have something magnetic to stick to them, which can include another magnet, but usually ends up being steel. A very cheap and simple solution for round bases is using washers. You will have to choose to have them be a few mm larger or smaller if you are trying to match metric bases with washers made based on English units, but that is a small problem.

A great advantage to steel bases is that they add ballast to the system which is great at reducing the incidence of miniatures toppling.

Games and miniatures stores may have useful stuff as well, especially those that cater to historical miniatures hobbyists. Warweb.com's accessories page lists some steel bases in various sizes; I have not seen them, but the cost is comparable to washers, so the pricing seems reasonable.

Making a flat square base magnetic

This is the easiest of all of the approaches. Simply buy some 1" wide or 1/2" wide magnetic tape and cut an appropriate piece then stick it to the base. You can also use a square magnet, but that can get expensive.

Round Games Workshop Slottabase With Magnetic Tape

Advantages: This method leaves the the door open for future changes; you can pop out the magnet tape with little difficulty if you tire of it. You can slap a zinc washer on the bottom to provide weight and stick that to a magnet strip.

Note: Magnets can stick to magnets, but you should test it first. Most magnetic strips do not stick very strongly to magnetic strip.

Disadvantages: Not as robust and does not hold in place as strongly as the putty and washer approach.

• I start with 1/2" wide magnetic tape, which is also about 1/16" thick.
• I cut the strip in half and glue one on the back of the other. I bend them a little bit here as I stick them together to remove the curvature from the original roll shape. This leaves me with a long strip of 1/2" wide, ~1/8" thick magnet material. This thickness just barely projects from the bottom of the base, providing a stable base without noticeably affecting the height of the model.
• I then use "The Chopper" a cool little cutting device that can be found in modelling shops.
• If I am chopping out a bunch at once, I cut the long strip into two strips, as The Chopper can accommodate a 1" cut.
• Set the spacers and chop away! You need two different spacings for the two different compartments in the bottom of a slottabase, 0.32" (5/16" or 8 mm is fine) and 0.22" (5.5 mm).
• Peel off the backing and stick them firmly into the base bottom.
• I then press them between two stiff objects, using about 30 or 40 pounds. You probably don't need that much weight, a few heavy books will just fine. This is just to insure the tape is well affixed to the base.
• Done! This holds my Tyranid troops quite rigidly.

Simple Washer Bases

The procedure is remarkably simple. Just glue a washer of appropriate size to the bottom of a mini. I recommend affixing a washer to a proper base, e.g. I use Games Workshop 40 mm large round bases and "fender" washers (1/4" inner hole, 1 1/2" outer diameter). Fender washers are ideal as they come in large sizes and have more material than a standard washer (smaller hole cut out of the middle).

Materials 5-minute epoxy Primer appropriate to the materials used. Base material - for these I used standard plastic GW bases. Steel-zinc Washers, 1 for each base. I highly recommend taking the base to the hardware store with you for a good size comparison.

Procedure Lay the base upside down Slather epoxy into the high points of the base underside. Tip: I usually hold the base in place by pushing down in its center with the tip of my hobby knife; the hole in the fender washer makes this easier. Smear a bead around the inner edge Gently lay the washer on top Gently position the washer in the center (it can be a pain to re-center it once it nestles down in a ridge, so be careful to center it properly the first time) Gently press down on the washer to insure a good fit (careful not to push it off center) This results in the following:

Round Games Workshop Slottabase With Zinc Washer And Internal Weight

Advantages: This makes a very durable, heavy base that is attractive to magnets.

Disadvantages: Somewhat permanent modification to the base (although you could always cut away the miniature from the base if you were really desperate for a change).

Cost: for 32 bases, I spend about 40 minutes, $2.50 in Milliput Standard Yellow/Grey epoxy resin putty, and about $2.50 in washers. That averages out to about 16 cents and 75 seconds per base. For the shot approach, the shot cost is negligible, but depending on how generously you use the superglue, the amount of superglue used can easily rival the cost of the putty. With the amount I use, the superglue can cost slightly more than the putty used. I am buying the little tubes of superglue I use for $1 apiece and I use up a tube with about ten or fifteen bases, depending.

Note: Squadron putty or other toluene based putties will not work well. They need to be applied and cured in layers, and will not adhere well to the washer.

Materials

• One 1" round GW slottabase
• Weight
  • Two pea-sized pellets of putty (I used Standard Milliput Yellow Gray), one slightly smaller than the other.
  • Cyanoacrylate glue and Shot - I used ~25 pieces of 7 1/2 Shot (0.095" diameter), but 0.090" diameter might be better*
• One 1" diameter zinc fender washer

* - I chose 0.095" based on the 0.100" cavity depth under the base, but over the text bas-relief the depth is about 0.090", so if you had 0.090" or smaller diameter shot, you could just fill the whole thing up as you see fit.

Note: If you use the putty, I recommend doing these en masse since you have to work the putty together anyway. I prep 1/4 of a packet of Milliput Standard Yellow/Grey and about 40 washers and bases. That much putty will generally work out to about 32 bases, but that is variable.

Clean Base & Washer

• Trim and file excess plastic from the slottabase
• Clean the washer and slottabase with soap and water
  
  

Putty Ballast

• Prepare the proper amount of putty and form two small pea-sized pieces, one slightly smaller than the other. You will learn by trial and error how big this needs to be.
• I wash my hands here to reduce the propagation of putty residue.
• Mash the putty into the two side compartments on the underside of the slottabase. The ideal piece of putty would not quite fill the compartment and would just barely break the bottom plane of the base. This will allow the washer to stick to the putty without having so much putty that the washer cannot lie flush against the washer. If you see that there is too much putty, pull it out, reduce the amount, roll it up again, then insert it back into the space.
  
  
  
• If you do the above correctly, there will be no excess and you are essentially done. If there is any excess squeezing out of the base, clean it up. Note that the balls of putty do not exactly fill the space, that is to allow some room for the putty to flow into rather than squirting out from the edge of the base.
• Wash your hands again to rid yourself of the putty residue.
• Let the putty cure and harden

Shot Ballast

• Place the base face down on the table
• Apply superglue to the area where you want shot to go; put it down in a thin layer that just barely wets the surface for now. In the case of 0.095" shot (Size 7 1/2), shot over the lettering in a slottabase will sit a little too high for the washer to lie flat against the base, so put superglue everywhere else. Some slottabases, like that for the Genestealer, do not have lettering and can be filled completely.
  
  
  
• Hold the base over the container of shot.
  
  
  
• Grab a pinch of shot and gently drop them into the glue areas. Try not to overfill it (honestly, with 25 pounds of shot for $20, you could probably waste 50% of what you use and never get done using it).
  
  
  
• Apply superglue to the shot. You are trying to completely wet the shot with superglue to achieve better durability, stability, and adhesion.
  
  
  
• Apply superglue to the rim of the base and any raised areas that might contact the washer.
  
  
  
• Press down a washer onto the base, being careful to center it well.
• Allow the superglue to cure
• Finished!
  

Putty vs. Shot - Which Ballast Is Best?

The only metric I really have is to test a top/front heavy unit and that is definitely the Genestealer unit from Games Workshop's Warhammer 40k miniatures game. That particular unit's base can be filled completely with shot, but for this comparison I took the worst case scenario and filled it with the same amount and position of shot that a slottabase with lettering inside would be able to accommodate. They both give the Weeblanid effect with a Genestealer. The shot seems to be the tiniest bit better, but the difference is almost nonexistent.

The method you choose will vary with your tastes and supplies at hand. I had to hunt for loose shot and when I found it (Carter's Country, I-10 and Campbell, Houston, TX), I could only buy 25 pound bags. So I have shot for life now. I will no doubt share the wealth with my friends since it was only $20 for the bag and I will be unlikely to use more than 10% in the next few years, unless I start loading shotgun shells.

Overall I prefer the putty method:

• quicker overall - it takes an average of about one minute to make putty-filled bases and you can do some steps in bulk, like mixing the putty. With the shot approach, you have to go from start to finish for most of it and a bulk or mass assembly approach is not really feasible.
• does not require a supply of shot - this is a small annoyance, but storing pounds of shot is aggravating.
• is cleaner, simpler, and easier - superglue can be a pain to work with. The putty-filled base approach does not involved foul smells or having your fingers randomly stick to the bases.
• no significant cost difference - the cost of superglue rivals the cost of the putty.

Foam-Washer Bases

I wanted some bases wide enough and thick enough to facilitate grabbing the trees mounted on them without having to grab the tree itself.

Materials 5-minute epoxy Primer appropriate to the materials used. Base material - for these I used 3/4" thick Foamular extruded polystyrene Steel-zinc Washers, 1 for each base

Procedure A Cut out an appropriately-sized piece of foam Place a thin layer of 5-minute epoxy onto a washer and affix it to the foam Sand the foam as desired; I used 100 grit sandpaper then 220 grit sandpaper. Seal/prime the bases. Pick your poison... Latex paint - seal and paint in one shot. If you are going to use spray primer, you can seal the foam via your favorite method, such as thinned down PVA glue, Woodland Scenics "Scenic Cement", etc. I was in a bit of a rush and since I was going to flock the whole base, I decided to prime these without sealing. I ended up with decent results using Painter's Touch Red Primer (made by Rustoleum). Done! Procedure B Same as above, but a slightly different initial approach. I knew what size washer I wanted, and I wanted the foam to be very close to the same size. So, I glued the washers to the foam before cutting it out. I was using a hot wire so I was able to cut very closely and reasonably cleanly to produce bases very close to the size of the washers.

Modified GW Flying Bases

Why bother? The standard GW flying bases are notorious for snapping off at the top like so:

This alone is a decent reason to improve on the design, especially since it can be a real pain to safely drill out the hole without messing up a paint job. Other great reasons to make custom bases:

• Ballast - I use washer on the bottom to weigh them down.
• Magnetic transport/storage - again, a washer allows them to stay securely on a magnetic surface.
• Customized post heights - with the standard GW posts, it is very hard to keep Gargoyles close together or to push them in close for close combat without seriously endangering their paint jobs. By making your own posts you can design the posts to help accommodate the size/shape of your flying models.
  
  
  
• Versatility - because they are metal rods, if you want to remove them, you can more easily twist them out of a joint without snapping the rod. I plan to exploit that using CA glue so I can easily transfer the mini to and from a different rod for painting.