I used 1" sq thinwall for the longitudinal upper mounts, and 5/8 (IIRC) emt to connect it all together. Rather than welding it together, I brazed it: much stronger because it's a low temp process. Be carefull welding/brazing emt. The cadmium plating releases nasty gasses.
Here is the frame test fit my my killer:
I used plastic cutting board top and givi-case hookup hardware. Finished prod looks like this;
I had a set of Givi E21's laying around, and the fit on pretty well. Abused them mercilessly on dualsport single track rides and they always stayed on.