I am making a trench board. Currently have a 2' by 2' board made up.
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WIP shots in my gallery and in my PLog (link in sig).
One way to help eliminate range guessing is to offset your trenches on the boards. Although you may start the trench in the middle (say) of one edge, jink it around on the tile so that it does not stay at a fixed set of distances from the edges. Mine bends around fairly randomly, while maintaining straight sections of trench:
Edit: I would also advise you to use pink foam, and to use a thickness greater than you might expect. By the time I added duckboards to the bottom of my trench, I had to build up the walls higher than I was expecting so that the men inside actually had cover. I was using a thickness of about 1 inch - 1.5 inches. I would suggest using 2 inch thick foam so you can really sink the trenches in.