ok, heres an example of how i might roughly do it.

you want solid areas around the engine mount, wing mount and tailplane mount. i use ply for the engine mount and balsa around the wing and tail, then build the shape up with sticks, then make a template of it- just draw around the whole lot, including the cutouts for the wing and tail etc.

sheet both sides now, sand back the edges until you meet the stick frame, lay the template on top and cut through to reveal the cutouts for the wing/tail.


as for thickness, i would use 3/16 for the insides of the fuselage like the ply/balsa/sticks, and then sheet it with 1/16 balsa.