You could make the hole in the frame that the U rod is going through a slot. Then weld a small tab to the rod that will only fit through the slotted hole long ways. Pull the rod out until the tab clears, twist it so the tab won't go back through & voila, pin held out to pull the ball. No more need for the big U.
Nevermind. I figured out what you're doing right after I finished typing. LOL The big U through me off but now I realize you're cutting that long side down to just long enough to hold pin out while pulling the ball. Nicely done, sir.