Dresden, what scale and what purpose are you building your model? And how truly "light" does it need to be? For example, if you were building a 1:144 scale model for actual RC naval combat, I'd say either plywood or fiberglass hull and balsa skin as per club regulations. If, on the other hand, you were building a 1:200 scale model for display purposes only (no RC, does not need to float) I'd say don't worry about weight and use whatever material you want. Or if you were building a 1:72 scale model to fill with helium and use at your local RC airplane club, I'd recommend cardboard and paper and say "good luck". In short, give me a bit more information so I can help you better.