Another thing to consider is how are you going to mold the tank? You mentioned a wood skeleton, but that won't really be sufficient to use as a mold for the tank. You'll end up with wavy/sagging walls. Now, if you sheeted it with plywood and used that as your build-up, you would be in decent shape, but if you take that approach then you're already 1/2 done with a traditional plywood tank...