I did a very accurate measurement of the support ribs a few months ago, but I left the data on another computer and I won't see it again until this weekend. I remembered something about the ribs being a little over 2 inches, but now I remember that was the length from one side of the hat channel to the far side. You are right about the size of tubing inside the channel itself.
Darn. The 2" square tubing was $125 for 49 pieces, each 59" long. It seemed like a really good price.