Welded and ground smooth (well, sorta)
I leaned it up against the door so you can see the cargo box bottom attached to the pivot unit.
A closer view of the pivot unit - you can see the chainring and plate welded together and two 1/4" bolts attaching the cargobox bottom to the pivot unit.
Here it is, back clamped in the vise.
A side view across the cargobox bottom.
Next up are those outside pieces to hold the wheels ...
and then the vertical corners.