it looks hokey, but if you're really worried, you can put a hasp and padlock inside the bed, on either side. you'd have to put a 90 degree bend in the long part of the hasp, to put it on the inside corner. what hood do you live in, where this is a serious concern?

* master lock has one pre-bent. the tab would go on the side post, the slider on the tailgate itself, as close to the top as possible. one on each side, your stuff isn't going anywhere. LOCK