An O-ring won't work too well on a narrower band like with the flight ring, but with a cuff-style ring it strikes me as being pretty effective. And a well-equipped hardware store will have them in so many sizes you're bound to find one that fits.

For narrower bands, if you don't want to buy a purpose-made spacer, get some thin (say, 1/8" or maybe 3/16") rubbery plastic tubing and cut a short length off, then slit it the long way and slip around the band. This, or a proper spacer, will tend to roll off the ring when you take the ring off, though.