wow, 2x2x3/16" tubing ought to be plenty stiff! of course, you lose stiffness with length... I guess I'd just have to take a look at the drawings and do some calculations from there. Likely, the 3/16" wall thickness was chosen to give you less than 0.001" deflection at the maximum expected loading. (at least that's how I would do it if I were designing a machine )