Looking at these. I bet a bit of a mod with some aluminium strip, cut from an old beer can with old pair of skissors, and you would have a power pack.
Make up about 6, and you change packs instead of fumbling with batteries. Old Ceag or Oldham headset and lead, fit a Retro 2 or Miles Moulding special, endless possibilites?
Of course, not waterproof, but on the back of helmet, unless you plan to go diving?
Food for thought
My avatar is a poor likeness.