All the internal "tubing" is either 1/4 or 3/16 hardwood dowels, the window framing in the greenhouse is basswood or spruce, depending on which one it is. I doubled the airfoiled former that is the attach point for the wings, and everything that was lite ply, I re-cut using the same thickness hobby grade ply (5 plies instead of 3). I pinned the window frames into the wing roots, and added dowels on all the vertical cabin supports (only three, two on the left and one on the right. I did not detail the rest of the cabin; I had intentions to, but haven't gotten around to it, doubt if I will, but my 1/4 scale will be detailed to the max. Sig said the plane's avg built weight is 6-1/2 to 7-1/2 pounds, but mine came out to 10 pounds because of the mods, scale landing gear and a couple of repairs. It's still a floater, and a .65 Saito flies the hell out of it. I haven't figure out what I want to use on the 1/4 scale yet; it will be gas, though; I'm looking at a RCS or ZDZ since they are so narrow and won't have a carb sticking out the side.