These guys they ship
http://www.perfectfit.com/15269/Carpet--Padding.html
Get extra to do the kick panels and along the door bottoms as well, I got the felt pad at a carpet place but had to search for it. Pad is hard to cut, hit the edges all around with a propane torch or a wood burner to keep it from raveling, wood burner works good for making holes as well.
Leave it out on a hot day on a flat surface, not the lawn, as it will kill it. After its installed use weighted bags to hold it down till it cools. I used bags of cement for another project. Use paint on contact or spray, what ever you prefer just not too much as you want to take it off someday. For the back, I glued the pad to the metal, added the carpet, there is enough room to tuck it under the double cab lip. You can use Luan to make corners or finish as you please.
Some Home depot and Lowes sell marine grade indoor out door in gray and black as well.