If you can find a working "Wonderlight" headlight, your search might be over. It's a headlight/flashlight unit that uses a nylon clamp that the rectangular flashlight slips on to.
I used to have the thing on my bike in the early 80's. Nice thing was, the rectangular headlight unit fits my front shirt pocket, so I could leave my bike locked up outside classes without worrying about someone stealing it. At the same time I got to use the headlight as a flashlight at home.
There's lots of better headlight/flashlight units (much brighter, lighter and smaller) out there today, but I think the Wonderlight was so popular back in the 80's that it would be the appropriate thing to put on a classic ride.

Chombi
84 Peugeot PSV