When the box is empty the latches do up tightly with the right latch having only a mm of play. With weight in the box it has about 1/8" of space. As a quick fix I inserted a twig to tighten it up but what is everyone else doing?
I have not had that problem, sounds to me like you have bent the bracket on the bed, not allowing the latch to grab tight when you fasten it. I would try using an adjustable wrench and tighten it up tight against the bracket on the bed and flexing that piece a little. Be careful not to break anything or go to far. Again that's what I would do. Good luck