True, the only wood prop I've ever bought was for my Cub which has a spinner nut. To get back on topic and contribute something, any spacer a person uses needs to be a high friction material so the props don't slip. A smooth washer is not sticky enough generally. An easy fix though is to get a sheet of the open mesh sandcloth for drywall. They sell it in a roll for plumbing too. You want the kind that you can see through; it looks a lot like screen material for your windows. It's very coarse and is rough on both sides. It will bite into the surfaces you put it between and won't let them slip.