I think ya'll got it covered. lol
Basically in a regular cab, you have to go wide and skinny. It's not optimal, but you do what you gotta do.
You can kinda see how I built the box on page 5 of my old cardomain page
Sadly I have no measurements to give for that box, as I just measured and built that box to fit every bit of room I could find. But if you want a milder size box, here's a lil blueprint I made up:
One good place to start is by removing the tire jack, and put that into a tool box, or something. That frees up alot of room. The magic number for subwoofers is 6". No more than 6" top mount depth unless you want to really cramp the legroom. Sub diameter is really whatever you want to do, but 10 or 12" is the easiest.
Any more questions, just ask...