After reading various threads, I happened across a handy little battery box attached to a Fenix bike light which takes two 18650s and we've ordered half a dozen for stock.
It looks like this:
đŸ”—Personal-Album-2-Image-97717[linkphoto]Personal-Album-2-Image-97717[/linkphoto][/link]
A little bit of bodging to fit a new connector and it's usable with a StenLight:
đŸ”—Personal-Album-2-Image-97718[linkphoto]Personal-Album-2-Image-97718[/linkphoto][/link]
It's pretty much the same size as the current StenLight battery pack, but about 30g heavier (this will vary depending on which cells you use).
It's rated IPX6 water-proof, which means it's not going to let water in if briefly dunked, but submersion for longer periods isn't feasible. However, I've left one submerged overnight with no signs of water ingress but the cable entry may not be water-tight.
The obvious down-side is that it doesn't have charging circuitry built-in so if you were to connect it to a battery charger in an absent-minded moment, it's not going to end well...
If anyone wants one, I've popped them on the webstore:
https://www.abcstore.co.uk/Excellent-Stuff/Fenix-battery-cases__p-230-726.aspx
Hello again darkness, my old friend...