Dang.
been wracking my noggin trying to figure this. At first I thought a horizontal hole and pin through the tubes and jack points but drilling those accurately now would be super difficult.
Thought about tapping a hole in the bottom of the jack points for a smaller screw to augment things. Still seems bodgy.
But you nailed it: a key hole with the roundy part at the front, the slot just wide enough to slide above the ball of the jack point. You slot these on and then feed the bolts up and tighten. If the bolt ever backs out or breaks the part stays locked on as long as you pull.
The only problem: the plane of the tops of these brackets want to pass directly through the balls. Raising them up 3/8ths to make the key slots work might be more than I can achieve using my rudimentary tools and skills...
Yours is definitely the optimal design though. Thanks, Carlos.