I'm following this as my neck is not as able to let me know what is behind me as well. I have a bar end mirror like the one above on my newer bike, but I don't think it would look right on my older bikes.
This one is cheap enough that if I didn't like it I wouldn't hate throwing it in a drawer or box in the garage.
https://www.amazon.com/UNIVERSAL-VIN.../dp/B075SDCY3L