Purchase a swivel hasp latch lock (a latch on a hinge with a rotating anchor) and some bolts. To determine how many bolts you need, examine the latch lock to see how many holes the latch has to secure the latch to the box. Purchase one threaded washer for each bolt as well (the washer should fit snugly onto the end of your bolts).
Open your toolbox and remove all the trays and tools from the inside. Set them in a safe place until you can put them back into the toolbox.
Set your latch lock and anchor against the toolbox to see how well it fits. You need to set the latch lock vertically so that the latch rests on the bottom part of the toolbox and the hinged side of the latch rests against the side of the toolbox lid. The anchor is the part of the lock that the latch catches on, so position the anchor where the latch can easily reach.
Poke a pencil through each bolthole on the latch and anchor to mark where you will locate the bolts. Remove the latch from the side of the toolbox, leaving nothing but the marks where you will install the bolts.
Drill starter holes into the side of the toolbox where you will install the bolts. It will go straight through the metal of the box. The drill bit you use to make the starter holes must be smaller than the bolts you will use to install the latch lock.
Set the latch lock and anchor back over the toolbox, lining them up over the starter holes. Ask an assistant hold the latch lock and anchor in place while you insert the bolts into the starter holes, securing them both in place.
Open the toolbox and screw some threaded washers onto the ends of your bolts. This will prevent your bolts from simply slipping out of the toolbox.
Replace all your tools and shelves back into the toolbox. Close the lid once everything in place and secure the latch over the anchor. The anchor will have a hole in the center where you can slip a combination or key lock through, effectively locking your toolbox shut.