Now that's a cracking bit of engineering there Nash, looks really well!! A word of advice though buddy.
Being an ex-engineer I make racks for my bikes, and have done for years. You really need to connect the front of the rack to the grab handle by some means, or the arms at the last angle welded will fracture due to metal fatigue as the box bounces up and down when you ride.
Just ride behind another bike with a box on, and watch the bugger bounce, then you'll see why!!
Best of luck with it, and good job!!