I have a 1930 model A 2 door coupe and when I replaced my top I took measurements and made a cardboard template of the curvature. I took that information and went to the junk yard where I found an old station wagon that had a good top with the ribs. I cut it out, took it home, did some hammer and dolly work around the edges, spot welded it skipping around to avoid warpage, ground it off, applied filler, block sanded, primed and painted and it looked like that top was installed at the factory. Wow, that was a lot of work. Hope that gives you some ideas.