Great idea, well executed and very cost effective! Would also make a great camera mount! What size ball is that? Where did you find a flange to fit? What size is the stud on the bottom of the ball? Is that alum plate behind/around ball?
Just a "standard ball." I don't own a trailer so this is what I got when I walked into the local O'Reillys. Bought the flange at an Ace Hardware. The idea and plans for this came from a student I was in a class with who made one. He said it came from an earlier edition of Woodworker Magazine. I just copied his work! Sorry that doesn't give much information. Yes, those are aluminum plates on both sides of the vice of which semi-circular holes are cut and the ball is fitted between them and clamped with both nuts that you see.