Well you're just on fire now!
I can see problems with that design with the holes not lining up just right & some outside interference (your hand) needed to make sure it locks fully into place... somebody I know has designed and built the ultimate failsafe coupler, maybe he'll share his secret with you for a 100% hands free lock into place every time. You're on the right track though, but you just need to redo a small part of it.
