I went buy the local big-box home improvement store and picked up a blank sign made of corrugated plastic (these will be located with the signs that say "Garage Sale", "Car for Sale", etc) for $3.98. I also got four basswood square dowels for .93 each. Total cost: under eight dollars.
I gathered up the models for a typical-sized unit:
I arranged them so I had the (#number of models) (width) x (depth) and marked it with a straight edge:
Then I cut it out with a utility knife:
Next I measured the wooden dowels, and cut them using my hobby saw & mitre box so the corners would be neat:
Then I glued down the dowel rods with PVA glue, setting them on some newspaper to dry to catch drips. (Actually, I used some old glossy mailers in the hopes that the glue wouldn't stick so badly). I "squeegeed" some glue into the cracks with a spare piece of wood to fill gaps:
Then I backed them with some scrap cardboard:
Then I made some for the artillery units, with cutouts for each model:
Then they all got a coat of primer:
For painting, I mixed up a somewhat close match using mostly craft paint, as it's much cheaper than hobby paints:
Then, the edges got the same basing treatment as the rest of the army:
Next: Final product!