I had the same problem when I wanted to use my Dry direct drive 3 x XML 2500 lumen flashlight on my bike. That flashlight uses 3 18650 and almost the size of a 12oz coke can. It was a monster and this show how big it was next to a P7 and Geoman battery pack and Bikeray IV.

If you have a shink tube heating gun, then you can make your own DIY saddle and buy a bar clamp. I made my out of PVC pipe where I cut a pipe that has about the same diameter as my flashlight. I cut the pipe in half leaving two tabs in the center. Then I used the heat gun to bend the tab over to create a hook for a Oring to hold the light down. If you don't have an heat gun, you can still screw some 10-32 bolt on both side and loop the oring on that. Then you bolt the saddle to the bar clamp. Here is a picture of the DIY saddle. I added ssome stability by putting some velcro at the bottom of the flashlight and the saddle. This help with keeping the light from moving forward when I push the clicky switch on the tailcap. Here what it look with the flashlight mounted on the saddle. This is what it looks like on the handle bar. To remove the flashlight, it involve nothing more than taking the Oring off and peeling the light off the velcro from the saddle.

Eventually I did not like the length of the flashlight nor the runtime and wanted to mod it so it is shorter and takes external pack. I hack the light, installed a cap, switch and some wire and this is the final result with a MS900 next to it as a size comparison. I even made it work on my helmet because I was able to lighten the head unit to 210 gram, but the battery has to me in my pocket.