I am going to build in a trunk car soon. The seats don't fold
If your seats dont fold then consider taking the deck speakers out as crappy as that is, and maybe add port flare ends only where the speakers went if they are round speakers. I love deck speaker ALOT! But i'm sure it would be for the best, that's what i am going to do soon.
If you build the box in the trunk then put together the bottom and front/back peices already and what not and leave the top/sides off so that you can rotate it into the trunk, i cannot explain what i mean but i tried.