Sometimes Coroplast or cardboard won't do
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As sheet steel and wood are relatively inexpensive, were I wanting sheet steel skirts I'd have a go at making them myself. There'd be few simpler shapes. I'd also be prepared to have a few goes at it before I was happy with the results. Re. the linked page, one of the guys featured in the video on hammer forming - Ron Fournier - wrote a couple of books on metal working. They are standards and worth a look to get the basics.
However, I think it will be easier to form a pattern (for each side) of the skirt shape, make a fibreglass mould and the part(s) in fibreglass from that. MDF would be a suitable and appropriately cheap material for the pattern. The pattern will need painting to a gloss finish.