If his goal were purely cheap, this is not the way to do it. He made several choices that are more expensive, and require strong materials than he needed to.
That amount of metal, the weight starts to be the dermining cost. Whatever shape you want it in, is essentially free. At what, $10 a pound, aluminum ain't cheap. If you could knock 20lbs off the frame, you just saved $200.
If the design were aimed at being simple, he would have done the fuselage from three bars. Which could be done and kept the same geometry. Which would have used less square tubing, and would have not loaded any beam in bending.
He made his design choices based on what "looked right" to him. not what were good choices for weight, or cost.