You can make a very simple system by mounting a servo next to the fuselage side, and having a pushrod stick out through a tube in the side of the fuse. Make its lenght so that when you flip the switch the pushrod retracts fully into the fuselage. You can then put a rubber band on the pushrod where it exits the fuse, and attach the other end to a wing hold down dowel. When the pushrod retracts, the rubber band will come loose, dropping any load you mounted underneath it.