About five years ago I got twenty sheets of these heavy duty galvanized roofing panels. They were used, had screw holes in them, and only cost me I think $50 for all twenty sheets. They are 11' long by about 3' wide. At Home Depot I think they sell for about $36 each, last time I checked. Again, these are the heavy duty panels, not the lighter duty ones that can be had for about half the price.
Just about three years ago, I decided to make a border planter box along my long driveway using them since I hadn't used them for anything yet.
I first considered cutting the metal into 12" sections by 11', and then building a frame out of wood to screw them to. The more I thought about it, the more I realized if I could bend the sections, they'd make a perfect "canoe", and would need a minimal amount of extra material to keep them squared up like a planter.
The only materials used were the panels, seven of them bent in two places, about 220' of cheap 1" X 1/2" furring strips, some caulking to make the seems look better when the panels overlap, some screws, and some paint.
I think the pictures will explain it better.
Never seen this done before this way. They are holding up great, although I didn't do a great paint job on the furring strips so the paint is flaking off now.