fiberglass tanks
Fiberglass tanks seem to be pretty common in the boatbuilding industry.
Epoxy is a much tougher resin, and I imagine that it is, or could be rendered fuel proof with an appropriated addative or coating.
Instead of balsa, you could make up a plug mold out of expanded polystyrene (styrofoam) and lay up your tank with successive layers of cloth or matting and resin. While another poster suggests that mat roving is stronger (he is right) the woven polyester cloth is easier to form into compound curves because you can stretch it along the bias (diagonal to the weave) much farther than you can stretch it in either direction along the warp or woof of the fabric. The roving is sprayed out of a chopper gun and has no "grain."
There are also boat repair shops that use chopped matting---that's a deal where chopped up fibers and resin are sprayed out of a special tool similar to a paint sprayer to build up successive layers.
The beauty of the styrofoam is that once your get your tank all laid up and nice and smooth like you want it, pour a little gasoline in it and the foam will turn to liquid and you can pour it out. CAREFUL, you now have what amounts to napalm---nasty gooey stuff that burns and sticks.
I thought about punting and just building a decorative shell or case around a metal tank.