I built one myself with some spare parts. I love it, to the point that I wear my helmet for short trips when I otherwise wouldn't bother.

It's modeled after this one: https://www.efficientvelo.com/home/safezone

The key component is the flexible, stays-where-you-put-it arm. Safezone is repurposing an industrial product called "coolant hose" - a 1-ft length costs less than $5. Epoxy a small mirror to the end, and attach it to your helmet with zip ties or velcro straps.

Here's mine: https://imgur.com/a/a9H7bXq

I tried some helmet mounted mirrors, but they moved around too much for me, so I switched to this eyeglass frame mounted mirror:

http://travellingtwo.com/resources/take-a-look-mirror

It's simple to clip on, and stays put once it is, I've been using this mirror for over 5 years and feel uncomfortable riding without it.