I had a desire to keep ammo out of the hands of children so they wouldn't be tempted to "play" with it.
It should go with out saying, but I'll say it anyway. Because I will be drilling into the side of the canister, the canister will no longer be water-proof. This wasn't a problem for me put it might be for you.
Step 1: What's Needed
1 - .50 cal ammo can
1 - 3/8 - 16 X 1 universal clevis pin
1 - universal hairpin
1 - 1/2" drill bit
1 - 3/8" drill bit
1 - 15/64" drill bit
1 - drill
1 - rat-tail file
1 - padlock with 3/16" diameter shackle
1 - bench vice
1 - Dremel
1 - end mill bit for Dremel
1 - #83 O-ring
Step 2: Locate and Drill: Canister
With that out of the way, let's continue.
1. Close and fasten the lid. Make sure that everything is lined up and looks good.
2. Insert the 1/2" drill bit into your drill and insert it through the hole in the clasp. Make sure that the drill is on the level on both the horizontal and vertical planes.
3. Lightly run the drill to score a location on the canister. Do not drill through the canister at this time.
4. Switch to the 3/8" drill bit and drill a hole through the canister using the mark created in the previous step as your center. As before, make sure that the drill is on the level. You may find it easier to start with a smaller bit and move to successively larger bits until you get to the 3/8" hole required for the clevis pin.
5. Use your rat-tail file to clean and de-burr the hole make in the canister. If you have a Dremel, this is a good time to do some clean-up with it.
Step 3: Locate and Drill: Clevis Pin
2. Slide the o-ring onto the clevis pin all the way to the canister.
3. Insert hair pin through clevis pin in hole closest to canister. This will ensure the pin doesn't fall back into the canister when shutting and locking the lid.
4. Shut the lid of the canister slowly dropping the latch over the end of the pin. Check for fit. If, when you drilled the hole into your canister you weren't exactly level on the level, you may find your pin points one way or the other. This isn't the end of the world, but you'll need to use your 1/2" drill bit and bore out the hole a bit bigger. The head of the clevis pin will allow you to have a bit bigger hole if necessary.
In truth, I wasn't able to find a 1" clevis pin at my hardware store so I bought a longer one and used a hacksaw to cut it down to size.
I then used a bench grinder to clean up the end. Longer than 1" though and you won't be able to close (or open) the lid.
4. Locate on the pin where you wish for your lock to be. At this point, we'll need to enlarge the hole on the clevis pin to allow for the width of the shackle of the lock. Remove the clevis pin from the canister and secure it in your bench vice.
5. Insert 15/64" drill bit into drill and slowly drill through the desired location on the pin for the padlock. Depending upon the holes in your clevis pin, you may find it handy to use a smaller drill bit first and work your way up.
6. Once complete, reattach assembly to the canister.
Step 4: The Lock
I used a Master Lock 131Q with a 3/16" shackle.
I suppose you could use a smaller lock with a smaller shackle, say an lock for luggage, but I felt this was not as frail as those locks look.