Thought I'd post some pics of a coupler lock that I welded up from scrap metal that I had lying around to fit a puck lock that I had purchased years ago. The only new part that I had to buy was a hitch ball from Wal-Mart.
The pics are pretty self-explanatory but you may not see that the ball is welded in place & I drilled a few weep holes in the bottom to drain any water that may accumulate.