I think your problem is that the prop hub needs to be a larger diameter to match the width of the propeller at its hub. You don't have enough surface area to grip the prop well, so it slips and comes loose on you. You should also cut in some grooves to grip the prop better. The prop washer should be larger in diameter to match the prop hub size too and more thick than a regular washer. Even a gas engine RC conversion develops too much power for using a couple of nuts with the prop in between them. The prop hub can have a wide end with a tapered or stepped smaller diameter rear part though.
You can see here where I made the prop hub adapter larger in diameter and used a big washer like spacer for the prop washer too.