I have 1 (soon a second) PSI platform one 12s that I plan on putting into a 4th order box in my trunk that I am going to port trough the rear deck (I will be cutting a hole through the rear deck so I want to be sure the box port is right lol).
Currently I'm thinking about a box 40 wide by 15 heigh by 18 deep. This ends up being 6.25 cubes approximately before any discplacements. I don't know what the exact discplacement of a 12 inch sub is, and can't remember what I used to calculate before. That's not the big discplacement issues anyway. The port will be mostly internal, but a bit of it will be external because I want it to actually fit up to the hole I'm cutting in the rear deck. I can cut up to an 8 inch square port through the deck, and from the top of the box to the bottom of the deck is 4-6 inches depending on where the port would fall (slightly sloped floor). I can adjust the depth of the box but the width and height are constrained by trunk size. So I have some flexability to adjust for the possible partially internal port
David at PSI recommended 1 cube sealed and 1.5 ported for these subs in there respective box types, but I don't know if that corresponds to 4th order as well (so I usedthose numbers for my rough calcs). I really don't know much of anything when it comes to port calculations.
I also have some concerns as well about the rear deck porting, I really want to do it because I don't have a ski hole and I hate trunk rattles. I am concerned about possible damage to the rear window seal/window itself having all of the sub energy firing directly at it in close proximity. If anybody has done something like this and can ease my mind or confirm my worries I would appreciate it. The car is a 2007 Mazda 6 sedan.