Other plans have been made for the other enclosure so I started building this tonight. It's a transmission line tuned to 35hz with a .85ft^3 compression chamber and it's kind of self loading. It models up well and should fit in the trunk just fine.
Should sound nice, get loud enough, and get down low with ease:
This one requires much less wood than the horn:
Once this dries, I'll route the mounting baffle out and then it should all slap together:
Hopefully I'll have it drying tonight and can route the outer baffle and get it installed. At the very least I should have it in my room to get a listen.