Fenix sells a bike flashlight mount for around $15:
https://www.fenix-store.com/fenix-al...se-bike-mount/

Drawbacks are:
- Having to remove the batteries from the light and recharge them every time you ride with the light. Most bike lights you just plug into the charger. Not a big deal if you only ride 3 times/year in the dark but if you ride often it becomes annoying. I personally try to store my bike next to a charger so I can charge the lights without removing them from the bike at all so they're always charged and always with the bike.
- Sometimes the beam pattern that's good for a flashlight is bad for riding. It really depends though, and some bike lights have bad beam patterns as well.
- More work to attach and detach from the bike. Better bike lights you just slide on and off the mount.