Robert, here's how I did it on my Boxer 60:

Lay out the outline of the aileron , both top and bottom....a template makes it easier to get them identical. THEN, draw a line 1/4" on either side of that line. You're going to cut the surface here and line both the opening and the aileron itself with 1/4" balsa. When you make the cuts, angle the knife blade toward the hinge line; you'll square up the foam after you get everything apart.
After the 1/4" linings are glued on, tack the whole thing back together and plane/carve/sand it all to shape. Cut the appropriate tapers for your style of hinging, install the hinges, and there you are!

Scott