sounds like 4 cubes or more would be a tight squeeze getting it into the trunk without building it inside the trunk. if you could do it, outer dimensions of 32"W, 18"H, 18"D would give you about 4.5 cubes gross with a double front baffle. less sub displacement is about 4.2 to 4.3 cubes. less port volume, depending on tuning freq, will be about 3.5 net. I figured on around 37 hz with 66 sq in of port. 16.5 tall by 4 in wide, 28 in long. that is almost 1 cube port displacement. it'll end up probably a little small for a btl 15 but i think it you make it any larger you won't have any room around the box. if you make it so large that the trunk is essentially sealed off from the cab you will lose output that way. my numbers here aren't exact so don't build off of them but they should give you a pretty good idea of what to work with. good luck fitting the enclosure in the trunk. i really think you will have trouble unless you go in from the back seat though