I have 1/4" (6.35 mm) square spars top and bottom with a 1/16" (1.6 mm) shear web wherever the leadout wires aren't in the way. Rather than the aluminum tubes Mike and Paul use, I have some carbon knobs on the inboard ends of the spars in the wings. The spar caps are reinforced with carbon for about 10 cm at the wing root. I can't find any pictures or good drawings.