Over the last several months we have been experimenting with the best way to make these rib sections for the horizontal. It is very much a trial and error process. When these were mass produced they were able to essentially stamp these things out with no problem. Without that type of machinery around, we are forced to work every single piece like this by hand.
Fiberboard works well for building ribs like these. Use two sheets cut to the shape of the rib with holes drilled through them, sandwich a blank between them, fasten them together with bolts and wing nuts, and use a mallet to work the flanges over the sides of the form.