Here's the bottom bracket welded onto the main tube to act as the box pivot. I had to weld-in (close) a 1/2" hole left over from where a tube connected to the BB.
This is with the pivot mechanism mounted.
You'll notice that it is not a 90° angle to the main tube.
The diagrams and fotos that I've found show it at a slight angle ... probably so that when there is weight in the box, the rear of the box doesn't drag on the main tube. This angle on mine is the width of the Sharpie marker line (no more than 1/8" wide). I drew the lines to make the "fish-mouth" cut outs for the BB. After clearing out the excess metal with a 4-1/2" grinder, The BB was vertical. Then on only one side, I ground to the back of the Sharpie line. You have to file each side between the fish-mouth cuts to get a line that will allow the BB to seat. And this was the resulting angle.
The next questions is 3/4" tubing or 1" tubing for the bottom box frame?
We'll just have to see how it all works out.