first boat. 3/4" mdf sealed enclosure coated just like the OP running a 12w6. built in spring '08 and sold boat in summer '12. used it under the helm as a foot stool most outings. box was still solid and believe it or not, that boat came back up for sale this spring at prestige watersports(where we traded it in) again and guess what. same 12 under the helm in the same box.
last tige z3. had a 13w7 that was installed 4 years before we purchased from a guy in oklahoma and system was installed by a shop that has a pretty decent rep amongst the boating community. in same locker as op's box. bought in '19 and sold in '21. box was vented and built out of mdf.
still solid the day we sold the boat. heard this spring the kid that bought it is still using same sub/box.

this is just as widespread as the discussions to only use marine equipment with conformal coatings on the boards. thousands upon thousands having no issues but many still like to push that it's one of the 7 sins..
and oh ya... our '12 tige that had an exile zld in it? still running as of last fall. I've been waiting for David at Earmark to buy me lunch. said it would last 2 years tops...