I cut my rear speaker deck from the interior panels of what I believe was a older Kenworth tilt over cab semi tractor. The panels are large in size meaning that they are wide enough to fit across most rear decks. The panels I used were black vinyl in a rolled an pleated pattern. It saved me the hassle of finding new ones to match the cars original motif and frankly it gives the car a bit of class for the rear deck.

http://www.homasote.com/WhereToBuy/ Homasote is the fiberboard you need to use. Other than that, if it's covered with any material, use your local auto upholsterer for the material and use 3M adhesive for applying. Or paint it if that's the origional....