We started with the wings and tail of a HobbyZone Firebird Phantom park flyer. The fuselage is made of two strips of 1/8 x 1/2 balsa stick sheeted with 1/32 on either side. We didn't bother to draw a detailed design. First, we laid the top and bottom strips in the curved shape we wanted with some guide blocks on a building board, and added some cross pieces in between. Then we added one side sheet, removed from the board, trimmed the sheet, and cut and inserted a noseblock. Finally, we added the other side and trimmed, and sanded the corners.
(I learned this fuselage construction method from an issue of what must have been Radio Modeler magazine back in the 1970s. And it's been years since I've done any model airplane stuff. Let's see how far we get this time....)
When first test flown, the CG was too far back: it had little or no tendency to adjust its pitch attitude as a function of its speed, and would either dive and crash, or climb and stall. Part of the problem is that we used steel ball bearings, rather than lead shot, for the nose weight, and we ran out of space in the nose compartment.
We are still testing. Stay tuned for more details.
By the way: notice the price of the whole Firebird Phantom is only about $50, while the wings and tail alone cost a whopping $17.50. Whassapwiddat? If they can afford to sell the whole thing -- motor, RC, fuselage and everything -- for fifty bucks, then these two parts can't be costing them more than a buck each to make.
So, note to self: next time, build up the tail surfaces from balsa and Monokote. The hinged elevators on the store-bought foam surfaces are a useless pain anyway, and the tail surfaces don't need to be cambered so they should be pretty easy to make.