Yes, if using light ply for the side maybe go with a lamination of glass or even maybe thinner 1/32 or 1/16 birch ply, the more resistant the cabin sides are to twisting and bending the less carry through structure you'll need. I think you could get a rigid structure made even from 1/32 birch ply laminated to both sides of 1/8 balsa center for a small plane like this. I've used that to make a structure for a fuselage that had two big doors on either side that could open, that structure carried the loads around the doors, worked good and didn't weigh a whole lot.
The best thing is to try a keep the plane light as possible, then you won't need as much strength. Although parasitic drag will remain the same , the lighter the plane the less induced drag the wings will develop.