porting from the bed to the cab?

Head down to a place that sells truck canopies, they have a flexible metal reinforced rubber gasket for this purpose, they caopy dealers use it when they put a canopy on the truck and remove the rear window from the truck, it has a slip fitting on both edges to go over the metal edges that you will be cutting out. Make your box to go in the box, either resin it or have it sprayed in rhino lining (truck box liner spray material). If you are jus trying to put a couple of small ports into the cab, you will not get near as much outpout into the cab as if you do a blow through, this means that you will cut out two large holes, one behind each seat for the output to pass through, leave some metal in the center of the truck to support and strengthen the rear window area.

I would usually build this as two separate boxes, box one is the sub box, box two is a "funnel" than attaches to the front of the sub box and passes through into behind the rear seats, if you do this make sure you leave at least 1.5" all the way around to allow for the flex of the body and box of the truck, the rubber gasketing will keep out all rain etc. By using this method, all of the output of your subs will be channeled forward into the cab, you will not be losing output around the leading edge of the sub box where it comes into contact with the truck box. Make sure, real damn sure that this box is bolted down with a minimum of 6 bolts, it is a big heavy missile in an accident.