A simple extruded aluminum L-angle piece with the forward edge ground to a knife point, then flush-riveted with countersunk holes to the trailing edge of that wing would probably be fairly inexpensive - about 30 minutes worth of work at a sheet metal shop. There's enough 'meat' at the trailing edge to drill and countersink the required holes(underside on the wing, topside on the angle), having them flush riveted and the forward edge of the angle ground down would likely give you the cleanest installation possible. Granted, you'd have to drill it out to remove it, so you'd want the proper height of the angle calculated beforehand.
Edit: Forego the rivets and countersinking on the wing itself. Have the angle countersunk and the holes drilled in the wing threaded, and use countersunk screws. Easy to change the flap height if needed.
The Gurney flap, from the source.