This is a final layout for this army display.
What you see is a picture frame with the glass and inserts replaced by hardboard glued to cardboard. And by cardboard, I mean I took the back off the picture frame, traced the shape on a cardboard box, and then cut it out with a box cutter. The hardboard I bought at Lowe's Home Improvment in one big piece and I had them cut it down to 2x2 sections that were more manageable.
I drilled the holes in the hardwood with two different bits - again, both available at Lowes. The big round one is meant for drilling holes in doors.
The holes in the foam were a bit more problematic. I resorted to tracing the models' bases with a pencil and then using my foam cutter stylus to to melt out the circle. Not optimal!!
Next step is to fill the cracks and glue on some sand and scatter. . .