I've been using a glasses mounted Take-A-Look Mirror for over 25 years. It's metal and brass. If you don't wear glasses, you could start wearing safety glasses. Or I think some people mount them to their helmets.
On my glasses, at least, I merely have to slightly twist the mirror into place at the beginning of my ride and it stays put, even after 10 years of use on the current one. The previous one is at least 25 years old and I keep it in the shed with the bikes for quick, impromptu rides.
