Rectangular tubeThis is for a frame design that gets wrapped. (Kind of like a wing.) Although there will be some lateral forces, most of the force will be vertical only with lots of vibration. The steel would be 14 gauge and the aluminum would be 1/8" thick. The orientation of the shapes would be with the 3" dimension being vertical. Also, comparing a 6061 vs. 7005 alloy would be nice also.