My son and I recently made our own rain ring per a thread I found. We mounted on the underside of the screen top and it's working great. Might be an option that would not have the rusting issue??
Sorry for the delay, I had a crazy weekend! I'll find the link to the instructions but really simple. I think it took us 20 minutes total. The outer ring is 18" across and provides very good coverage.
Thanks, the OP made it really easy. I was going to put the mist nozzle in there as well but it puts out such a nice little rain shower that I don't feel it's necessary.