> "I'm wondering why not make new ones out of sheet metal rather than cardboard?"

That is exactly what I did with my Quantum Syncro. I made them from aluminum sheet material. It is easy to work with and way more durable. If you still have the cardboard ones in decent shape, another trick I have done is to spray them with truck bed coating. This is sold in spray cans at just about any autoparts store. It gives them a durable coating
that adds some slight stiffening.
hope this helps,
Craig
the Gorge VW club