Several options, much will depend on your set up.
My BC has a pocket with a D-Ring nearby. So, I have a bolt snap with a small section of line attached to the light. Light goes in the pocket, line is small enough to fit under the zipper when the pocket is closed. I attach the bolt snap to the D ring, put the flashlight in the pocket and zip it closed. It's out of the way and secure. I can actually operate the light without removing the bolt snap from the D ring. But if I were to use for a more extended use, then unhooking the bolt snap would be needed.