Some random thoughts. For a true glider high start you may be able to just reinforce only the center section of the wing. For warbirds I've simply glued bbq skewers to the foam spars. It seems to help a lot. Depending on how thick your airfoil is (assuming you have dihedral in the middle of the wing) you can get the skewers to go full length. Alternatively, I've been using 1/32 plywood a lot. You can cut it with a scissors. This could work as reinforcement glued to both sides of the foam spar.
