Ahhh, the ubiquitous ammo can.
My Scout does not lock and I need to carry tools and emergency gear. Solution: I got the normal size ammo can (M-60?) well, you know that good all-around size. I bolted one of these guys inside my engine compartment (lots of room on the Scout). The handle/latch mechanism has a hole in it - drill a hole in the body of the box "through" the hole in the latch; take a big bolt about 3" long (or other piece of metal the size of your hole) and grind a flat spot on it opposite the head of the bolt - drill a hole thru the bolt at the flat spot (the flat spot just keeps the drill from spinning off the surface. You can now insert the bolt from the inside of the box, close the latch (a little tricky but not bad) and put a padlock thru the hole in the bolt. The waterproofness is compromised to the degree that there is space around your bolt, but unless you are submerging the truck, this is no prob.
Take a much bigger box that has a different type lid (the lid has lips that extend past the top of the box) - cinch the lid down tight - drill thru both the lid and the box at the center point of the lid (halfway down the length??); do this twice, on directly opposite sides width-wise. Take an old tire tool (not a 4-way, the kind with a pointy end and a socket end) that is a little longer than the lid is wide, again grind a flat spot and drill a hole thru the tool. The tire tool goes thru the lid and box, and comes out the other side (wifth ways, not lengthwise); padlock and you are good to go.
One more hint: If you bolt these to the vehicle, try to bolt from the underside so the nuts are locked inside the box. Then either loctite or just booger up the threads. Also get some fender washers (large O.D.) and use on both sides.
These things are solid and in such cramped spaces in my Scout that the only way to rip off my stuff is to either have a grinder handy or somehow chain the boxes to a truck and rip them out (and you better hope I can't sight in on you with the '06 from my deer stand)
Sorry for the long post, but I have been waiting for the opportunity to share this.
Good day.