For a test pattern I use electrical conduit. Very cheap and easy to bend/test fit in the car if you have odd /complex shapes.
On symmetrical parts like a hoop, start at the center and move out.
If you are ever using seamed tubing in your bender, put the seam on the center of the inside radius.
Use one piece hole saws. Hole saws with separate mandrels wiggle and distort and will not last long.
I use hole saws in my milling machine for notching tubing. If you have that luxury, try using it. You can use the same hole saws for what seems like forever, because with solid mounting, and auto feed, the hole saws don't take a beating.