I am making a dual ported 12" sub box. I have a 12' Dodge Avenger and putting in trunk. I was just wondering what is the least amount of inside space of the box it can be? I am trying to just set it in the back back of the trunk with room to put my amp on top of the box with some like 1/4 spacers or something if I can.

box specifications are completely dependent on whatever subwoofer(s) you use.

you can't just make any size box and throw any sub in it.