I have been looking for a cover for my RMR that fit like the rubber bikini covers I like on scopes. I ordered a set of covers from Perfect Plastics for $10 and they are exactly what I was looking for. Don't know how durable they will be but they seem very well made and they do an excellent job of protecting the RMR.
isn't the RMR sight 'water proof' already? I am putting one on my G40 MOS pistol, and bought a kit that has a waterproof membrane that fits on the mount.