I found some aluminum washers that are about 1/8 thick and knurled front and back at a local hobby shop that were designed just for doing this. Not a clue where the LHS got them, they bought out old shops and estate sales. I also have wood spacers with crocus glued onto the front and back that I made from different thickness ply that I use. These work very well but stand offs behind the engine or mount is a much better idea. The wood washers tends to compress when tightening the prop. I have never had a spinner come loose but I can see it happening.