While I've been writing all this I've also been making ribs. After 100 ribs you get very good at it. That many ribs makes quite a stack of ribs.
Now that they're done I did one more experiment. I wanted to know if this would work with 2024-T3 aluminum instead of the 5052-H32 we first selected. The 2024-T3 is much stronger but also has more spring back and I didn't think we could form the channel walls square. The 2 ribs at the top are 2024 and the rib at the bottom is 5052. Clearly the form block needs more curve to allow for the spring back of 2024 but otherwise it works fine. It does take much more force to make the ribs. The channel on the right is 2024, again no problem with the shape. Also there is no evidence of cracking along the corner radius even looking with a 10x glass.