RE: plywood fuel tanks
If one has made up his mind to go "plywood" then I think Chris has hit on it. Years ago I worked on a project for one of the big pineapple companies. The problem was how to ship pineapple juice in bulk but in a container that could be palletized, moved easily and reused many times over. The solution was plywoood containers that "knocked down" (folded) easily and could be shipped, economically, back to the filling plant empty. (They didn't have to be cleaned all the time either.) A bladder was used and the program was very successful. The dimensions of the "box" was 4 feet cubed.