Instructables.com has plans for this bicycle handlebar window box.  If you’re lucky, you may also be able to find instructions for that quality bike light, too…because the parts for that almost certainly cost less than a simple handlebar light.
The best part of the instructions for the box are this:
“Get a crappy old pallet and saw it up something like this.”
It’s a good thing Instructables.com doesn’t have medical training articles.