Was looking at stuff in the garage the a while back and started thinking that my Ryobi flood lights looked like they'd fit over a pair of handlebars if only I could keep them secure:

Lo and behold, my wife found some 32" gear ties on clearance at Walmart just as it was starting to get dark early around here...

Didn't have much to lose since I already had the lights, so I decided to mount one and try it out:
Ryobi P721 - 2400 lumens & 3-4 hour runtime for < 0-img_20191008_204221868.jpg

From an initial test run around the neighborhood it seemed pretty secure but the beam was wider than I'd expected and worked well with the light angled down slightly to try to avoid blinding cars.

Was out again Tuesday with my son - we were supposed to join up with a group ride around 6, but he threw a fit and refused to go. Let him play on the playground till about 6:55 and started walking over to tell him it was about time to go home (sun is setting around 7:15). He comes over to me instead and tells me he "wants to ride his bike a bit". Had the light mounted up, so I figure why not.

End result was great - he rode a little ahead of me and the light was bright enough for both of us (he had a cheap 100 lumen flashlight I'd mounted as a backup, but it was vastly overpowered by mine). About as close to riding in daylight as I've experienced.

With the gear ties secure, I didn't notice much movement of the light even over rocks and roots. Checked the 4ah battery I had on the at the end of the ~50 min. ride and it still showed 3/4 bars.

In the end, the only real downside is the weight (all together it's close to 5 lbs for the light & battery.