I have RR's extenders as well and like them, but agree that the way the hex wells rust sucks (the plastic covers supplied don't stay in place long and will disappear). Also, they can be a PITA to tighten, since sometimes when you move the mirror it loosens the connection where the extender connects to the bar mount--you then find your mirror flopping around.
What would be great is an extender with a friction ball mount that lets you loosen and tighten it in 360 degrees (like a RAM mount for GPS, cell phone, etc.). Then you get unlimited visibility and adjustment, with no loosening/locktite issues.
Are you listening entreprenuers???
I'll add that early on, I cleaned out the rust with brake fluid and pipe-cleaners, and then made a little paper cone to shoot some black Rustoleum from a rattle can into the hex opening to cover/stop the rust. It took two people with blue locktite beforehand, however, to fully tighten the extenders. About six months and all is well, but I suspect that the extenders will loosen-up again.