Let's see if I understand what this ring is correctly. It is a metal ring of the same or similar thickness as the rotor where it mates to the hub. It sits on the hub and the inside of the ring sit snug against the part of the hub that sticks out and the stock rotor hole fits against. The ring fills the gap between this surface and where the larger Cobra rotor hole is.

This would keep the rotor centered and fixed. Otherwise there would be some slop (very bad) and stress on the studs.

So if this is the case the ring should be made of a durable metal (suggestions), a very simple ring shape with no other features (holes etc) and be of the same thickness of the rotor or a little less?

Pretty much a hub centric ring for the rotor to the hub as oppossed to the wheel to the hub.