Got one of these Hafny bar-end mirrors from Amazon for about $15:
I used to poo-poo mirrors until I started commuting semi-regularly. This thing is fairly small but very adjustable and stable, not much vibration and gives a surprisingly good view of the road behind me. My knee doesn't bump into it very often and I rely on it a lot - probably too much sometimes - to see what's approaching from behind.
https://www.amazon.com/Hafny-M0nday-...-1-spons&psc=1