Average three man blind is 8' and a four man blind is around 12', I was thinking a 4x8x4 and a 4x12x4 with rolling tops (we have the tops figured out) I was thinking: build 2x4 framing/sub floor and wrap with exterior ply, epoxy coat, tape corners and seams. There will be pressure from backfilling as well as ground water. I plan to anchor with screw type earth anchors. that's about as far as I am....
