Nice work there Stu.
Is that 20 x 20 X 1.6 wall thick box section for the rack, and 10 mm dia round for the pannier bars you have used there?.
Here is a thought for you: Would it possible make a base for the hitch bracket and drill two 8 mm holes in it, and then weld a 30 X 8 flat into the rack with two corresponding
slotted holes to utilise two 8mm (seat post) quick release skewers to fix it in place?