I am a great believer in the "air weighs nothing principle" so as much hollow as possible is the way to go.
Probably my most extreme attempt was a scale VTO EDF rocket/plane, a V2 with wings. It was actually built and flown right at the end of WWII. There is even a period video on YouTube of it taking off.
To work at all it was going to have to be really light so used planking over thin Depron 'ring' formers but each reinforced with 1mm balsa around the inner edge.
I also developed a way of building each fuselage section vertically as one piece.
Looks ridiculous to start with but once the top and bottom formers are in and correctly positioned with the four planks in place it permanently sets the geometry so the rest of the formers can be simply glued in place at the already marked positions.
It is then just a case of adding planks on alternate sides to equalise any forces.
All done in 2 mm thick Depron but the result in an incredibly light but stiff structure.
The tapered nose and tail sections were done in the same way although the tail was the most complex as it carried the 4 fins, the 70mm EDF with four big flush inlets.
With the heavy EDF right a the back the 1800mAh 4s had to go right in the nose along with the ESC and 4x5g servos connected as elevons and rudderons meaning all four tail surface acted for roll control to help counter the considerable torque from the EDF.
It weighs 23.5 oz ready to go with a take off thrust of 44.5 oz so it gets away pretty quickly with a 0.9 g vertical acceleration! Such a delicate airframe is speed limited so you have to throttle back just a few seconds after lift off.
The video is edited but the flight was nearly 5 minutes long.
More an exercise in lightness than a truly practical RC plane but the take off is impressive and somewhat surprising it flies very nicely.