Here’s my old faithful solution:
It’s a 750ml storage bottle with an intermediate section of cable made up to go between the battery and light, hole drilled in the lid with a grommet which keeps the water out and allows the cable to be fed in/out to allow more or less cable to mount the light, means I can carry a spare battery pack (4×18650 type) and some other bits and pad it out with bubble wrap or old socks to minimise rattling.
That’s done three winters now and not let a single battery pack get wet…
you could probably do something similar with a wide neck bottle, carefully drill the centre out of the bite valve perhaps and add a bit of silicone perhaps…