Cokin P size adapter rings plastic?

Do they make any cokin p size (doesn't have to be cokin brand) adapter rings that are plastic? I'm needing an adapter ring that is close to the size of a cokin p, but needs to be a tad smaller. If they don't come in plastic, how hard is it to grind down the metal ones?

I have a bunch of P size adapter rings but I have never seen a plastic one.
Grinding down a metal one would be easy but mounting it on a lathe could be awkward at best!

What, specifically are you trying to do?

I'm wanting to use it on an old Ambico bellows shade.