I mean rubber seals and o-rings exist. If I remember correctly the law doesn’t demand easily swappable batteries, but rather them to be replaceable at all. So just use screws to hold the backplate in place, it could even look somewhat cool like on a Royal Oak Watch.

Samsung used to make the “Active” lne of Galaxy phones which were waterproof shock resistent and had removable backs and batteries and a way for the phone to detect if the back was properly sealed.