Check out your nearest salvage yard. Many full size trucks and vans used fiberboard interior/ceiling panels that had some sort of finish on the exposed side that could be painted over to match what was originally under your dash. You can also get new fiberboard from your local hardware center but that stuff is usually too stiff if you need to bend it and needs to be sealed before you paint it. My 250 panels originally had a sound absorbent padding glued on the back.
You can use fiber carpet padding when you finish yours.

Happy Motoring,
Mark

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DrDKW