re: Using corrugated sheet metal for above ground garden beds

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My concern with sheet metal is heat.

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Metal won't get any hotter than any other non-permeable material, like plastic.

Plastic objects sitting in the sun get just as hot as metal.

As for the reason metal "feels" hotter:

Say you have a metal block nd a plastic block sitting in the sun, they both get to 125F.

When you touch the metal, because it's very conductive, the entire block conducts heat to the area of the hand trying to maintain an even temp profile in the block, which means your hand is cooling the entire block, while receiving a lot of heat.

With the plastic, since it is a poor heat conductor, your hand locally cools the area it is touching very quickly, and the rest of the block (parts you aren't touching) maintains its 125F temp. But because it's a poor conductor, it has trouble making up the heat lost in the area of your hand.

In the case of wood and concrete, they are permeable, so moisture can evaporate out of the soil through them and provide a cooling effect.