wouldnt the car seat be pretty easy to take out? at least easier than a sub box? ive never had to do that lol.
i have seen a SQ car with a 15" sub under the spare tire. not sure what he did for an enclosure. it wasnt meant to get loud at all though. if you wanted to do something like that you would need to cut the floor and make a fiberglass box under the car. a hell of a lot of work for bass. the bright side being it would look completely stock unless you look under the car.
the next easiest solution i can think of would be to make a false floor that the box will attach to/be a part of. on the back, next to the seats, you have a big hinge. now you can lift the box up far enough to get to the spare tire. you could even make it all downfire with openings of grill cloth in the false floor to let the sound back up. this way you dont need a removable top and no one can see what you have. and to hold the box up you could go simple with some wood stands or you could do shocks and not have to lift all that weight.
OR, duct tape the spare tire under the car, on the roof, on the hood, wherever you think looks best