I feel like I'm probably missing something huge here, but isn't 4.5x4.5 steel square tube a standard steel tube size? I've used it all the time for machine frame weldments, the corners are rounded too.
4.5 x 4.5 x 3/16 wall:
http://www.metalsdepot.com/products/...ml?page=sqtube
4.5x4.5x.12 wall:
http://www.tottentubes.com/square-steel-tubing
I kinda assumed it was a solid steel tube, didn't expect them to make it bent sheet metal. I guess maybe the weight savings from not having the bottom wall?