I used my battery powered hot wire cutter to shape the foam and then cut out a small base of Masonite (a type of hardboard invented in my home state of Mississippi), beveling the edges with a wood rasp. The base was spray painted with a sand camouflage color and the walls of the emplacement was painted with a sandstone color craft acrylic. The two were then glued together with Elmer's white glue. Good enough right now for the first play-test on Saturday, they will eventually have interior wooden slat walls and a terrain application on the outside and the "floor" of the interior. I anticipate that I can use these for just about any period from the mid 1700s to modern times.
Here's another view from a slightly higher angle.
The Krupp gun and the Egyptian gunners are Ral Partha figures.