Bedliner on the Seat Box
I took the seat box out and had the underside coated with spray-on bedliner. I also had the undersides of the floors plates and the access panels lined. It was done by the local "Ultimate Liner" guy - he seemed a bit more professional than the local Rhino-Lining people.
I know lots of people have had this done to the interior of their trucks, but I didnt want the look of bedliner inside, plus I spent so much time cleaning double-sided tape off of the seat box that I didnt want to cover up the nice yellow paint.
I really like the result a lot. There is probabaly 20-30 pounds of material stuck to the panels, and it deadens them up very nicely. If I had though of it in time, I would have seriously considered doing the underside of the bed when it was off the chassis.