take the pull start rope out of the starter housing and stuff it in the cylinder through the spark plug hole (you can use another type of rope, but the pull start rope is usually handy), use this to keep the engine from turning when tightening the prop adapter, and I just do that with the prop mounted to it for more leverage... a little blue loctite on the crank threads for good measure

and yes, one set screw to keep the ring from turning