Mill Frame 01
I put together an idea of a mill frame using the rails from ladder cable racks. The red sections would be welded on 45-degree joints, and the yellow parts represent steel plates welded into the corners for rigidity. The dark gray parts would be bolted in place to allow the frame to be disassembled and adjusted.
This frame is modeled larger than necessary to exaggerate apparent stresses on the rectangular tubes. The bottom pieces are modeled as 36" from front to back. In reality, those won't be larger than 24" (with corresponding down-scaling on the other members).