Good news for Flume owners who are currently stuck with the sealed battery cases -- according to Reddit (not my own firsthand experience), Flume is now selling a case that enables you to use your own batteries.
It sounds like it's perhaps gone back to the original case design they were using at first with the Flume 2 (that open-case / easy-to-swap design was replaced by the sealed version).
I just ordered a replacement battery and this is what they had sent. According to their support this is the new user replaceable battery. It is wrapped and I asked them how to change the batteries and they said to contact them for directions when it dies. So maybe take that with a grain of salt.
Cool, that looks like the original battery case -- that's nice to confirm.
IIRC, just carefully cut & peel away that shrink-wrap label to expose the battery cage.
I use a mains-powered battery eliminator with mine, so it's been years since I've touched that pack. Once I put the replacement batteries in, I think I added a wrap of electrical tape at each end to help ensure everything stays snug (similar to what the shrink-wrap is doing now).