The fuse cover is rarely the issue we find. Seems silly to focus on this. I'll bet you $100 to charity that this is NOT what has caused a failure on your battery.
Yes, Tesla did flip this to the bottom of the pack at some point in 2015, but this was for ease of replacement (don't have to remove the battery pack) and nothing whatsoever to do with moisture ingress near the fuse. Any time I've seen a fuse cover rusted/damaged/etc, it's been due to some other issue (moisture from inside, damage from unqualified repairs, etc). The actual issue is almost always the side vents along the rails that fail and allow moisture ingress into one or more module chambers, almost always near the center of the battery pack. (There's a few reasons why this in the location, but perhaps a story for another time.)
The fuse covers are actually usually very well sealed. Even more so than the rest of the battery pack cover. The failure points tend to be the side vents, followed by the through-bolt aluminum fasteners that have an o-ring at the top of the pack. I've yet to find a car where a cell group or other related component has failed without a moisture related cause.
We always update the side vents, or we'll transplant an entire pack of modules from a v1.0 or v1.5 enclosure into a v2.0 enclosure if needed, although generally the side vents are more easily replaceable than a full transplant.