Heat gunned some styrofoam and then glued it to an MDF backing, hit it with a base layer of acrylic caulk and brown/black acrylic paints, drybrushed with successively lighter browns over a day or so. Got it to where I'm happy with it, really like the texture the styrofoam emulates with that sort of bogged down muddy terrain that's seen a lot of foot traffic through it.

Next up is making modular grass to lay over the top of it once the stuff arrives in the next week or so, plus other bits of scatter terrain. The idea is that I can lay this down as-is for a muddy battlefield or put down modular bits to convert it to a muddy-ish field with plant life etc.

Got another two of these unpainted that I'm thinking I'll do up in a gray stone and brown-red stone with additional texture from sand to create different surface textures on the other boards, plus a bit extra from foamcore glued ontop.