Hello im planning on hooking a Skar DDX 10 inch with about 1.7 to 1.8 cubic subwoofer box. I plan on making a box underneath the rear seats on a f150 but im going to lift the seat a couple of inches. My question is ive never understanded how big or long port holes should be for a box. I've used PVC pipe on 3 boxes already and they've sounded good. So would a 5 inch or 4 inch diameter pvc pipe be good for the box ? And im not sure about the lenght of it.