I've used this one for many years:
Cycleaware Reflex Bicycle Helmet Mirror
https://www.amazon.com/gp/product/B0...rch_asin_title

The mirror is plastic (scratchable) and pop off sometimes (so I used a little silicon glue to keep it where I wanted it). Easy to move to different helmets if you have a second glue on mount on the helmet.

Those bottle cap mirrors (linky no worky) have been known to gouge peoples faces in a crash. Not good.