well, my suggestion would be to do as I did. Build the wings as if you were going to sheet them(ribs, formers etc), foam inside between the ribs, fill any and all cracks or gaps, and glass it. This could work well either making a mold this way, or just to cover the wings. Also look into (if going this route) carbon strands, or sheets(small sheets). The strands could be used to join the two halves together and make a stronger edge. And carbon sheeting in proposed stress spots could streingthen the surface missing the balsa sheeting. I uderstand the length issue, as mine each are 6.5' long. On my wings, the only balsa is for the"I" beams(built the plane as to sheet it, not fill with foam to be made into a mold.) The rest of the wing was all mahogany ply.
Good luck with your decision..