I used the front half of a 1985 Toyota van roof turned backwards on my coupe. The curvature is perfect in both directions and there are no ribs to deal with.

If you try to use a simple piece of flat 20 gauge steel, you will need access to an English Wheel to curve it slightly or you will need to heat-shrink the edges with a torch, hammer & dollies to make it fit. Otherwise it will look sunken in the center and will "oil can" like crazy.