Making a fiberglass fuse that's both lighter than a balsa structure and stronger will be a task. You'll need at least a few formers in the fuse to maintain the shape and rigidity, and it will likely have to be thicker than you might think. (I don't think one layer of 1/2 ounce glass would support the wing and tail structures for example.)

Common thought is that a glass fuselage can be built to about the same weight as balsa, but lighter is tough.
Dennis-