Ron,
That's a very cool way to do the top rib. I had mentioned that to my machinist but he felt that would be too slow a way to go for a production run. I also visited with him about doing them on a magnetic chuck. That might also work , but he decided to make up a holding fixture to hold them while they are being machined. It works well with a few provisions. The fixture must fit that exact rib, that means a seperate fixture for each different size. The milling cutter can't be forced to cut too much at a time, otherwise massive warpage and failures occur.
If I had to make one up myself, I would have it soldered to another bigger piece of steel to hold in the machine. I like your idea about slicing the tubing into strips with a 60 degree cutter. Please let us all know how that works out and thanks for sharing your projects with us.
Bob