Truck bed tool box ideas?

[jchicago]

Wondering if anyone here has repurposed something to use as lockable bed storage.

Right now I have a flatbed on the back of my truck (casually looking for a regular bed, but have a long list of better things on the truck to spend money on) and the PO gave me the plastic toolbox that he had mounted on it. Which I can't stand, it is the style that hangs on the bedrails, and the locks don't work. So looking for either a used metal tool box or something unique (60-70's period correct would be cool) to use, my only idea so far would be an metal Army footlocker. But it would need to be lockable and something long enough to carry my shotgun when I drive out to the range, and would like it short enough that once I get a bed it would be lower than the bedrails.

Any ideas or pics of what you got?