To the cargobox frame, I have added the outside support tubes for the wheels. These are large enough to contain 26" wheels. Designed for the use of front wheels only. I marked the 3/4" tubing at 4" and 37". I aligned the 4" mark with the arrow on the tubing bender and gave it a 45° bend. Then I cut the tube at the 37" mark, measured back 4" and put another mark to align with the arrow in the tubing bender for the other 45° bend. Before doing the second bend, I eye-balled the tube to make sure both bends would be in the same plane of reference. Rotated the bent end until it lined up straight, then made the second bend. Make sure the tube parallel to the box frame, is 4-1/8" out from the frame along the middle straight side. Also check by laying the tubing down on the frame and supporting it with a bit of scrap to make sure the outside of the tube ends don't extend beyond the cargobox frame. If necessary, you may want to tighten the bends a bit. Draw extensions of your frame line and cut off the tubing at an angle for welding to the box frame.
This definitely won't roll through a door now.