The best solution I have found yet is to get a piece of foam weather stripping, the 3/8" variety, and run that around the inside of the ring. First start off with a couple of 3" pieces on each side, then add until it stops rattling. My reasonong for doing it that way is because you can get the ring too tight and cause more problems.
But, I have done it on my headlight and spots and it has worked very well at a minimum of work and cost.