I had a hard time finding polycarbonate or POM plates at all when I was looking for something GK64-ish. I went with brass because I didn't want to spend hours scouring the internet for a piece of plastic.
I'm surprised to hear that POM is apparently more durable than polycarbonate. Polycarbonate is used for ski masks, paintball masks, riot shields, and can even efficiently stop bullets with enough material. If I want say, a protective laptop case/shell, I refuse to buy any that aren't polycarbonate.
I also had no idea that POM was the same as delrin. I remember buying an expensive delrin bolt for a paintball gun ... which I still have never used, many years later.