You might try a simple spring clamp with some wood blocks attached to the jaws. Put a string on it so it doesn't disappear into the bilge when you forget it's there. If placed just right, it'll drop right off if the shaft turns. It takes very little pressure to initially stop the prop from turning, especially a little 2 blade in the up/down position behind the keel. The noise of the prop turning is not only extremely annoying, it also is wearing on cutlass, tranny bearings, and seals for no reason.