I haven't gotten to that point on my truck yet, but when I am ready to start putting in carpet, etc. I plan to use half inch sound board wherever feasible. The stuff comes in a 4x8 foot sheet for around 6 bux at the local lumber yard and works great. It is basically the same material they use for acoustic ceiling tiles, but is a little more flexible (especially if you dampen it a little) It is a bit crumbly to work with but on projects where I have used it before I just spray painted it when I was done to bind the surface and that seemed to work fine.
My 3 cents (inflation, ya know?)
David
