You will need:
a piece of wood large enough for your chosen models to stand on
fabric scraps (some thicker material, some thinner/stretchy)
a glue gun and/or sewing materials
nails of various sizes, depending on size of models
cardboard (for larger model sizes)
Step 1: Cover the base with fabric
Cut out enough fabric to cover the top and sides of the wood. Place the wood face down on top of the fabric. Put a line of glue along the edges of the bottom and attach the fabric. Make sure the glue dries flat so that the board will be level. One way to achieve this is to glue and then flip the wood over and weight it.
(This step could probably be done by a staple gun, sewing, or other ingenious method. I just happen to have a glue gun).
To fasten the edges of the fabric, fold it as you would gift wrap and glue down. (You can see this better in the larger scale photo with the white fabric).
Step 2: Insert nails
Make sure that the space between each pair of nails is large enough to accommodate the models you plan to display. For my micro minis, I did intervals of about one inch. For my stablemates, I increased this to an inch and a half. You want the space to be large enough to accommodate different horses and perhaps horses wearing tack as well. If the space is too big, however, the purpose of the holder is defeated because a model that tips over will fall completely onto its side and risk damage.
Insert the nails so that they are secure, but leave them sticking out to about the height of the horses withers. If your nails are not tall enough, you can still make supportive dividers using cardboard (see step four). In this case, simply insert that nails so that they are secure regardless of height.
Step 3: Make fabric dividers
Each piece should be as wide as the distance between the two nails (below, 2.25 in.) and as twice as tall as you will the stall dividers to be. In the picture below, that height is equal to 2 x the nail height, but if you need dividers that are taller than the nails, this will need to adjust (see step 4).
To make a stall divider, fold the fabric in half and glue or sew along the two short sides to create a little pocket. Making the dividers the proper size for the nails can take some trial and error- you may need to reglue the sides to make it fit tighter, or cut along the long edge to make it the proper height. In the third photo below you can see how the pocket can turn out over large. You need them to be taut to work as proper stall dividers.
The hot glue gun works for this, but if you want it to be even neater, close the edges with a sewing machine and turn the dividers inside out to hide the stitches.
Step 4: Attach dividers
If you are making a larger scale and need to increase the divider height past the top of the nail, use cardboard to maintain a sturdy divider. Cut out cardboard as wide as the space between the nail pairs and as tall as you would like the dividers to be. Then slip your fabric divider pocket over the cardboard and then over the nail, so that the nails hold it up but the cardboard creates height and stability.
Step 5: Fill with ponies!
You can customize these holders in many ways. For example, if you have lots of time on your hands or access to a sewing machine, sew the pocket dividers instead of using hot glue. You can make fancy dividers in your stable colors, or even customize them for your favorite models. I made these holders with matching "stall" sizes, but you could easily create special stalls for larger models, foals, etc.