They are reall easy to do. Just pick a bay about 1/3 the way down the wing from the fuselage. Get some 1/4" bass wood or other harderer wood and glue a strip along the servo bay, 1/16" below the rib edge. Glue a cross piece across each of those strips and then put a 1/4" balsa across these ends, but flush whith the edge of the ribs. Cut 1/16" ply to fit this opening, clip the corners just a bit. Two 1/2" square lugs epoxied on end and a whole cut for the servo arm. A #2 sheet metal screw in each corner, cover and go.
I think these photos will give you the idea. Just position the lugs and the slot so you get full movement of the servo arm. Set them up about three inches from the hinge lilne

Don