Idea:

Put it in the open position as far as it'll go. Clamp it for now so it can't move while you're messing with it. Drill a hole in the corner of the top triangle thing and the moveable leg (same hole -- lower left hand corner in your picture). Run a bolt through the hole you just drilled. Either use a nut (for more difficult removal) or a wingnut (for easier removal) to hold the bolt in place.

Now the leg can't move and the holes should line up every time you open it.