i had this problem all the time with my s-10 but what i did is go buy some L brackets with one opening on each end of the L and bought one large metal screw and one short one of course your going to have to drill the long ones into the truck bed itselft and the short ones to your box. I drilled the short ones first because its hard holding the drill sideways and still put enough force into the box (not really but made it easier) as well as it keep it save from thieves... so two thumbs up for this.
SOHCivegra
great idea if your not a crazy driver like i am..

but tried it, and after two weeks or so the back sticky part started to come of the back. (unsticking) and i almost got mugged..