As for strength, some plastics would very good strength in tension even when quite thin, but they would not be as strong in compression as a traditional balsa/glass skin. That might not be a problem, as the same can be said of film covers such as Monocote, which really is nothing more than a super thin plastic. I would think that even thin PETG would be strong enough, say .020 or .030. But If you go through all the trouble of making a plug, you could make a mold of it and layup a fiberglass body. Another way to go, if there are no compound curves, you could cover it in .01 or .015 G10. Unless you were going to make a lot of the same plane and already have a vaccume former, I don't see that it can be cheaper or easier than traditional balsa/glass.