I love a lot of your ideas, I was thinking the easiest safest way to minimize imperfections in the wood is to not cut the plywood. So basically 8x4 sheet used for every panel, except sides. But I would be getting the sides pre cut from lumbar yard. I might ultimately not end up filling it past the 3 1/2 foot mark, but I think it will look hot as hell. I am thinking 2 windows pricing glass locally to give me an idea as to cost. For sealing I am thinking fiberglass all seams corners etc. then using either a max acr type sealant and then liquid rubber like the idea of flexible membrane inside. I will definitely follow ur thoughts tho, I just didnt know if 2x4 doubled up would beat a single 4x4