Foam core wing vs. built-up
Ben,
IMHO, if I were going to use a spar, I'd make the spar out of end grain balsa sandwiched between two 1/64" ply sides. The spar is set in the wing on it's 1/4" side and is as tall as the foam core. You would only need to run the spar out about 75% of the panel length. Place the spar on the C.G. or on the high point of the airfoil. Foam core wings built in this manner are incredibly stiff.
I have made several thin wings about 1/2" thick and 38" long. It's really to build a wing this thin with the proper stiffness and strength. Using the same spar as above (only1/8" thick) these wings come out very stiff. I'd encourage you to strongly consider putting the 1 1/2 oz. fiberglass cloth between the foam core and the skin. You only need to have the glass out to about 80% of the panel length. You will not believe how strong wings built like this will be, AAANNND, LIGHT. Rgds, Jeff.....