I have a pair of 12" xxx in a 4^3 box. This is all going under the rear seat of a quad cab truck so it is as big as it is going to get. I have it braced with all thread to save cubes and it is straight on 4 cubes for the pair. I am having a little problem with the porting. The subs will have to downfire with the ports facing forward but I have checked with a couple of porting calculators and have gotten different answers from them. How much port do I need.