Shortly after my uncle gifted me with his 20 year-old M3, I took it to have it serviced at Marty Forscher's downtown in Manhattan. As he gave the camera back to me (after calling his apprentices around to show them the original black Leica seal), he asked me, "You've been scratching your eyeglasses on the viewfinder, haven't you?"
"Yes," I said. "Well, you won't be scratching them any more," and he showed me a black rubber ring he'd attached to the metal viewfinder to protect my glasses. It's still there after 35 years. No trouble seeing the frame-lines either, I'm happy to say.
Interesting -- I just noticed that this thread was dormant for nearly 5 years and was just resurrected!