Midbass drivers are made to work in infinite baffle settings so no enclosure, if you need to do an enclosure do it as big as possible, guess 1/2 MDF will be plenty.
Midbass driver IB inside a vehicle though? You will need to separate the front waves from the back waves somehow, right? Aperiodic maybe, but that is pretty exotic for midbass and is still an "enclosure".
When you place a car speaker in the doors or rear deck it's IB for all practical matters, just need to check that there are no big air leaks around the speaker so that it works properly.