Flashlights are false economy.

Beam patterns and mounting/retention are inferior to purpose made bike headlights.

Every group ride I've been on where one or more riders used flashlights and makeshift rubber band mounts, someone's light fell off and endangered other riders. Every ride. No exceptions. I usually ride at the back to encourage stragglers and keep track of folks who voluntarily drop off partway through rides. Every group ride I've retrieved flashlights that popped off or inadequately secured tailights. Every damned ride.

Those rubber bands and aftermarket stretchy mounts will break. I've used them for various accessories and they all eventually break. Recently one of two I used to mount a video camera to my helmet broke. That's why I double and triple up on retention with those things. At a minimum, two rubber bands and a zip tie backup. I'll often use velcro or other mounts as backups too.

Those makeshift mounts are such hazards I'd ban them from group rides if I was ride leader. Either that or require backup retention using zip ties.

If you do go with a flashlight and makeshift rubber band mount -- I don't care how fancy the mount is -- do yourself a favor and use a zip tie as a backup retention.

Or just get a proper headlight. They aren't that expensive. In particular Serfas discontinued lights tend to be heavily discounted and are often outstanding values.

Even purpose made headlight mounts can break. My first Serfas mount broke after two years -- Serfas replaced it free. My Light & Motion Urban 500 headlight strap just developed a crack tonight. So I've backed it up with a zip tie until I can replace the strap.