One of my flying buddies built a 50" span Wildcat (profile) from blue foam and fiberglassed the whole thing. The fuselage, wing, and tail were built entirely out of blue foam. The profile fuselage built from foam is a good way to go, you just need some kind of skin or fiberglass to keep the foam from bending. You could do the following to make the foam profile fuselage stiff enough to support flight loads

1) fiberglass the foam

2) carbon fiber strips from nose to tail, both sides

3) skin with balsa

Also drop in hard points with hardwood or balsa at key structural points to support the wing and tail loads

Good luck,
Fred Cronenwett