I use a 3rd Eye Pro helmet mirror.
The adhesive patch is obviously designed for either a skateboard style, or seriously old-school Vetta helmet with a large, flat contact patch; so I use a couple of zip ties to hold it in place on my Bell Alchera.

I ride a lot of chip-seal, so bar mounted mirrors aren't good for me. I also have problems with my glasses fogging up or just getting coated in raindrops so I tend to take them on/off quite a bit, so glasses mounted mirrors aren't a great option either. The 3rd Eye Pro is easily adjusted but stays in place against even the fiercest headwinds and/or 50mph downhills, and it's large enough that I've used it not only for a rearview, but also as a "cosmetics" mirror when I needed to get a gnat which snuck past my glasses and into my eye.