I've used this Blackburn road mirror for over 10yrs.

It fits over hood and secures w/velcro strap. I don't notice the extra layer when cycling. It stays firmly in place and uses a glass mirror so I can spot vehicles 1/2mi away in good conditions.

I did try to get a eyeglass mount (take-a-look) to work for me several times, but always with some personal problem. I find the bar mirror works better for me as I don't need to twist my head around to scan behind me and can just glance in mirror without changing position.