I've been using the neck and cap of plastic bottles (water, pop, etc.) epoxied onto some 1/16" ply. and mounted in the wing with the cap flush with the surface, then cut a slot in the cap for unscrewing with a coin. Nickels fit snugly into it, and you can stack 4-5 of them in there if need be. This is on 1/2As, may not hold enough weight for anything much bigger.