I didn't build Tim's subframe design, but did something similar to mount the rear bumper and hang the CTS-V muffler I'm using. I used 1.5" square tube - 0.125" wall thickness. Probably overkill, but I didn't want anything to break if some fat slob (that would be me) stood on the bumper to wash the roof or something.

Keith HR #894

20190530_194124.jpg