Walmart sells cheap-o padlockable plastic cases. I've looked at getting one a couple of times, but in the end I just moving whatever it is into the super-cab. I always had thoughts of blowing some bolts through the bottom of the bed to secure it, keep it from 'growing legs' and walking away on me. My uncle and I even cooked up a plan of welding some threaded nuts to the underside of the bed. This way you could run some bolts through the inside of the case, through my bedmat, through the bed, and into the threaded nuts. This way the case could not be unbolted w/o gaining access to the inside of the case. Yet when the case was removed from the bed, there would not be anything sticking up from the inside of the bed.
Never got around to doing this though..
2003 Ford Ranger FX4 Level II - Silver - Stick - fuelly
2009 VW Jetta SportWagon SEL - Grey - Stick - fuelly
2001 Suzuki Bandit 1200 - Red
Mildly Obsessed With Photography