I use "Masonite" which is a 1/8" tempered hardboard available at most all lumber yards. Have found that this is a really great product....can be dampened on the rough side and formed into curves that remain when the product is allowed to dry. It's cheap by the 4X8 sheet (under $10.00), easy to cut and I've used it for headliners, interior firewall panels, and door and kick panels. I picked up a whole roll of automotive grade velour at a swap meet ($10.00)and with a little spray adhesive applied on the smooth side, you can cover panels like a pro.
On the interior firewall, I beat the cost of the expensive sound deadening products, by gluing foam carpet underlayment to the rough side of the Masonite. Had to make it a "two piece" but it's been in for 2 years and looks as good as the day it was installed, and it works!! The foam underlayment is available from the dumpsters at most carpet stores and also works great under rubber floor mats and carpet.
Dave
