Something like the sheet of plastic used in baking. Might even find a washer in the bins at lowes that is correct size.Make one out of plastic?
Nylon is pretty common for "plastic" nuts bolts and washers. If you find a thin one for sure use that, otherwise might need to spend some time with the sandpaper.
Wonder if the thin, flexible white plastic cutting boards would work? Not sure what material it is, but relatively thin to start with.
