It worked! There was some minor cracking on the surface, probably because I used a bit too much water, but that was easily solved with some firm pushing. It has dried very hard, and adhered to the board without warping it. It doesn't look any different to the photo I last showed you, so here it is. Thanks to FAUST for photoshopping the pic.
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Just needs some ground cover, foliage, and a vehicle and it should be great.