Here are the trenches after I've begun cutting them out. I used a very sharp hobby knife to carve and gouge out the trenches after marking out how I wanted them on the foam. Once I finished this time consuming task I realized that instead of using 1 sheet of 2 inch foam and carving into it, I should have used two separate 1 inch sheets and cut the trenches out and glued them onto the top of the first sheet. D'oh...

These are not great pics - they get better as the project progresses.

I cut the foam so that the interlock on it overlaps for several of the sections but I now regret this because it makes it harder to swap sections in and out which is the ultimate goal. Many such projects are a fixed terrain that can't ever be changed and I didn't want that.