Most of the riders I know use mirrors, they've all been riding for years and for thousands and thousands of miles, and they all use DIFFERENT mirrors. Some helmet, some eyeglass, some handlebar. Maybe there's some magic mirror out there that you haven't heard about, but quite likely, you'll have to try a variety and just pick the lesser of the evils.
My stoker uses a helmet mirror, but it's one that mounts on the visor, I believe, so if you don't use a visor, wouldn't work.
I tried one that glued to the helmet, and it vibrated too much. I tried a clamp-on style, but the helmet was tapered wrong where it needed to clamp, and it wouldn't go on at all. I currently use an eyeglass-mirror, but it needs a rubber band to keep it from going flying. It'll stay on fine unless you whack it with your hand while wiping sweat, and in that case, the rubber band keeps you from having to go back and pick it up off the road. So it's not perfect, but it's my choice of lesser-of-the-evils.
You may have to look into making your own if you just can't find anything at all usable.
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"be careful this rando stuff is addictive and dan's the 'pusher'."