You should just be able to make the enclosure size either really big or as close as you can possibly get to the airspace the sub will “see”. For example my trunk is about 13 cubic feet or so (a spec I got off the internet listed as trunk cargo volume) so I used that as my sealed enclosure size. Most manufacturers will list hatchback cargo volume with both seats up and seats down if they fold. You’ll want to use the seats up spec obviously. If you can’t find the spec a rough estimate should suffice.