Don't use any metal rings anywhere inside the Uwave. Even on the floor of the oven, if it is not in complete bare metal to bare metal contact with the bare metal floor,there will be circulating currents in the metal ring that will make it get hot, and you might get arcing/sparking also. A ring made of non-metal material should be ok as long as it remains electrically non-conducting. That means no spills, etc., which are hard to completely eliminate in a real-life kitchen.