I have had a "third eye" type bike mirror on my helmet for years. However, recently I have learned of some genetic predisposition to certain types of skin cancer which affect the skin on the face/head and have become more cautious about how I approach the sun. As a result, I have taken to wearing a protective "sun drape" - link used to illustrate what it looks like, but I wound up having to modify it because even with my helmet and the drape, part of my face is still exposed to the sun at times.
Anyway, the consequence of this is that when my face is fully protected from the sun the protective drape sticks out a bit and I can no longer see the third-eye mirror. I'm not okay with using sunscreen instead, so I have been experimenting with other ways of mounting the mirror, but have not been able to find any that work - for instance, mounting the mirror on the side of the helmet away from the sun results in my not being able to see the important part of the road due to my back being in the way, and all the mirrors I have tried that attach to the bike I can't see when I'm hunched over the bike for speed and in most need of a quick check of the traffic behind me.
One idea I had was perhaps a strip mirror under the top of my helmet's visor might work, but I haven't seen anything like that.