Funny story about that metal. I went out to my wife's uncle's place in the country to dump some yard debris in his burn pile and started talking to him about the best way to make a raised bed garden, he took me out behind his shop and he had a bunch of siding etc. from his company (that I work for) that burned down in 1981. They were 3' wide 31" tall panels that probably came from the soffits. I cut them in half with an abrasive blade to use them. I don't know how long they'll last but I've got a pile of spares. It's a lot thicker than regular roofing tin.
Line it with plastic. Keep the wet dirt off the panels?