Two lengths of metal rod or round stock of different diameters. The large diameter should be the same diameter as the trailing edge of the wing. The smaller diameter rod should be whatever thickness you want to make the sharp end of the control surface.
Lay the metal rods on each side of the control surface and use a sanding block to form the angle. The tapered control surface will have an accurate angle if the rods are used to control the sanding block.

Control line flyers use this method to form the tapered "Flaps" on control line stunt models where the elevator and flaps are coupled.

I hope this helps.