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 th
e bare metal floor,there will be circulating currents in the metal ring tha
t will make it get hot, and you might get arcing/sparking also. A ring ma
de of non-metal material should be ok as long as it remains electrically no
n-conducting. That means no spills, etc., which are hard to completely eli
minate in a real-life kitchen.