I use the Third Eye Hardshell Helmet Mount Mirror.with its plastic screw clamp. But I never liked the attachment to the helmets I used. So I've been making my own mounts for years. I take thin sheet aluminum, bend it to fit the helmet visor. (I wear prescription glasses, not contacts and visors simply mean life is better.) Then I fiberglass both sides with light glass cloth and epoxy. Drill 2
#4 holes in mount and visor and use screws. The mount will outlast several helmets so if I can find the same model next time, I drill 2 tiny holes and done.
I've also used the EVT Safety Zone mirror (Cycle Oregon; broke my mirror and that is what the Bike Gallery traveling store offered - also developed by a former Bike Gallery employee and Cycle Oregon mechanic). Those mirrors are excellent ,,, but huge and goofy looking. My far shorter mirror places it exactly at the best location I can put the EVT and being far closer, it gives me the same field of view.
I will probably never wear a glasses mounted mirror or handlebar mirror. I move around too much for a handlebar mirror to be useful unless it is convex like a car right hand mirror and I cannot judge distance with those mirrors, My glasses are simply too important (and expensive) to risk damaging them with a lever sticking out to grab things. (Those event are also hard on my nose.) My helmet doesn't care. Catch that mirror and it just breaks. Helmet's fine. The mount is fine. (And those Third Eyes are cheap. $11 on Amazon right now.)
Pet peeve - why don't the mirror and helmet people talk to each other and come up with a bracket like mine? $5 would be far more than it it cost to make and package. And if there was a system that good, more riders might start using mirrors. And the helmet makers that took on that standard might well sell a few more helmets. I don't have it in me to push for another new product and I don't have the need to have it because I have the skills and material on hand all the time to make them.
Ben