Todd, I don't think you need to wrap your flat piece around that 1" tube as much as you're thinking. I think I would put a slight bend in the flat bar just enough to be able to weld the end of the flat bar to that tube and bring the weld bead around the corners about 1/2". I think that would sufficient. I know I can bend 3/16" X 3" with a hammer and anvil (or other substantial support) enough to do this. Done it before.
To attach that plate to the white support in your first pic. I think I would cut a piece of 1/4" steel in an oval shape to fit and weld in (or to) the end of the white support. Before welding it in, I would drill a 9/16" hole in it on center and weld a 1/2"-13 nut centered on that hole. This nut would be on the inside of the white tube and provide threads for a 1/2-13 bolt to fasten the plate to. This way you would have a convenient way to remove the whole support assy. if needed................Bruce