How can I use my small flashlight as my bicycle's headlight?
Can you maybe use zip ties to mount the handle to the stem, then attach the flashlight to the handle when needed? Can be tricky to get the angle right.
There are also universal flash light mounts, though I’m not sure they’ll work for the rather large diameter of yours.
In any case, please make sure to mount it in a way which doesn’t dazzle oncoming traffic.