The battery boxes I've seen in FWs (like OP has) have sealed & vented battery boxes from MTS Products. The upper & lower halves are identical and one half functions as an intake and the other as an outlet.
There are various sealed & vented battery boxes but tend to be expensive. A few brands are Moroso, Quick Cable, Tow-Rax, Noco & Taylor. If you can't find what you want, you can always make one providing it's properly sealed up and vented top & bottom. If you do some googling there are lots and lots of ideas for building your own. No issue with joining the vent from two boxes. If you change the box location, you need the exterior wall vent min. 6" above the box(es).