There's a thread in The Other Place about a similar issue - it seems you can't get old spares from Peavey but there was quite a good idea on an alternative cover. I suspect that part of the problem with finding a replacement is that it's not really a box, it's just a cover with a fixed frame and removable lid, which is fairly unusual. https://www.talkbass.com/threads/replace-peavey-tl-five-battery-compartment.1549341/
Come to think of it, another alternative would be to get small neodynium magnets and recess them into the outer frame in those circles just outboard of the screws, and glue some thin steel (not stainless) washers to the cover.