I'm completely new to CNC and should be careful about posting in a 'sticky' thread but I didn't notice discussion about diagonal bracing for torsion. I don't mean to tech how to suck eggs.
Torsion results in diagonal forces so you want material on the diagonal. Where you see flat plates braced together with diagonal webs, then the webs greatly help with torsional rigidity - e.g you might see this in machine tool castings where it provides torsion stiffness but allows for simple moulding without need for cores. If your design lacks torsion stiffness then look to add material on the diagonal.
I remember this from some lecture almost 40 years ago. I also remember that it is much more complicated to calculate the stiffness compared to closed sections so a good reason to avoid in designs! I 'remember', but it might be an over-simplification, that non closed sections all have similar torsional stiffness as flat plate. So a tube with a slit down the length is a flat plate, not a circular tube, The detail of the end attachment may complicate this in reality but as a rule of thumb...