I use 3/4 or 1/2 inch vinyl tape on the bottom side of the wing. try to get a color that closly matches the cover on the underside of the wing.
Cut stripes about 12 inches long, the crease the tape gown the center of the tape and set the "V" part of the tape in the gap and press the tape against the trai;ing eade of the wing and leading esde of the control surface. Try not to get any creasses in the tape when smoothing it out.
I've used this method and seams to work for me.
NEDYOB