This isn't in my XR, it's a setup I used to have in my Frontier and with 4 12"s. The box was a custom box built to fill the entire back portion. The port was also custom built to give me a wide range. So it would boom when I wanted it to, but still have a nice punch when I listened to rock.
I was going to have them build a lighted amp rack on top. I wanted mine to be removable, hence the box instead of fiberglass. And yes the truck was dynamat'd; in fact it took 3 layers of the thick dynamat just to keep the roof from flexing.