Why not just fiberglass the stock cardboard piece? That’s what I’m doing to my 85 swb. Get it back roughly in shape, coat it with resin. Press the glass cloth, not chopped mat, into the resin, work out any air bubbles (white spots). Then topcoat with more resin until the cloth is fully wet.
If you don’t want the thickness of glass, use canvas or old bed sheet material. That way you can still run insulation on the cab wall and hide it behind the tool tray back panel. Looks stock if that matters on a 45-50+ year old truck that should have been modified decades ago…