[JesseK]

If I had to guess, I'd say it's a for a drive train that's similar to what 1629 did in 2008. They had 2 wheel pods that could steer left right. When both turned opposite of each other, the effect was a very tight turn radius (front rotates left, rear rotates right, causing the robot to make a sharp left turn).

There may be other considerations that I'm not thinking of, but 3x1.5 C-channel (0.2" wall for the legs, 1/8-3/16" wall for the base) is rigid enough for what you're doing there and also fits the 2.5" super shifter into the channel very nicely. It would also provide extra aluminum to support the rest of an FRC frame.