Cable lock. Quieter than a chain. I have mine mounted in plain sight on the outside of my M101A2, and it has never wandered.
Of course, the trailer doesn't spend a lot of time unattended, either. Usually if I have the trailer I'm going from point A to point B pretty directly. The only exception would be a trip to Lowes or a similar store where I'm getting a large load of something. Then it will sit outside while I'm in shopping. Still, no problems so far.
In the bed as shown it's fairly out of sight, so that would help.