i found some tar-carpet squares in a bin somewhere, which i cut to shape, and they are amazing, but i would recommend with carpet squares not gluing them down because it becomes impossible to clean the floor, and keep an eye on any potential rust spots. I also have some standard house carpet cut to shape which fits over all the insulating tiles and keeps them where i put them. I can actually hear myself think when i drive now.
If you are using something like "Q Pads", a brand I've used, get the metal warm with a heat gun or hairdryer and warm the pad, It will mold over about anything and stick well. The better the bond and fit, the less wicking of moisture you'll have.
I have a 40, but my wife and I herculine the tub and bought speaker carpet and put it on the roof. We are planning on carpeting the tub, but we are going to try to build something that could be easilly removed.
how much do you want to spend?.....Cascade Audio products makes some great stuff to not only sound deaden your rig, but insulate it too. I used several of their products on our truck and have been amazed at the results. They make an insulating mat made from recycled denim faced on both sides with an aluminum mylar that insulates and absorbs some road noise.
Here are some install pics from our rig of the insulating mat and some sound barrier product:
I used Reflectix-available in most lumber stores. It costs about $40, and there was enough for 2 cruisers. I adhered it with small amounts of weatherstrip adhesive and lined the floor and doors with it. I'll do the firewall and hood next. And I'm thinking about putting it under a new headliner. Probably not audio-grade but it's just a frickin' truck and it seems to help quite a bit.