Velcro is quite strong and solid once attached to itself, especially under tension. Depends exactly where you wanted the light, but I meant with a couple of loops you could position and clip the light almost anywhere. Maybe I’m visualising your setup wrong though.
Not sure exactly what you mean. My light does at least have a clip.
I was thinking of something like this:
https://www.passportcycles.co.uk/shop/accessories/rear-rack-bracket/
But maybe I could try using one of the giant safety pins I have..