[JesseK]

You could go with 3x1 tubing and angle aluminum for mounts to the transmissions. One angle aluminum piece on top of the 3x1 and one on the bottom, which should cover the 5" face of the transmission for mounting. The angle aluminum could be attached to the 3x1 via welding or via riveting.

Weight savings should be roughly 4.5 lbs assuming a 36"x27" base. Since that's 4.5 lbs of pure aluminum, it'd wind up being less expensive as well.