Disclaimer #2 this is my first instructable please be gentle with the comments ;)
Step 1: Why the Lock Fails
Step 2: Materials and Equipment
These are what most people have around the house, if you have a drill press, it would speed it up.
Screwdriver, hacksaw, drill with metal bits, small diameter cylinder file, regular metal file. Rivet tool with 1/8 rivets, tape measure.
flat top metal bolt with two nuts that fit.
A flat bar of metal about 11 inches long (cut to size for your box)
Piece of "L" shaped aluminum about 11 inches (cut to size)
Step 3: Time to Measure
Across the width of the box (inside the side flaps). (Guard)
Height of locking door (modified tongue)
How much room between the pull down hatch (where the mail goes in) and then lowest point where the locking tongue latches (Guard)
Diameter of your lock (guard)
These numbers will be used to cut the materials.
Step 4: Modified Tongue
Cut the flat metal long enough that it will pass the upper metal lip of the opening about an inch (you can use the old tongue as an example, but add some length) and about 5 inches from the lock to the lower middle of the door. You will then create a square hole to attach it to the lock. This can be done by marking a template on your new metal with the old tongue and drilling it out. Use the small file to make the hole square. You will then drill a hole to accept the bolt.
The tongue should screw onto the existing lock. Using the bolt, level the tongue as it rests on the door (see the picture) use two nuts to lock in the screw.
It should turn freely with the key and hold the door locked. If the the door is loose, either try to bend the locking portion of the metal towards the door or put in a spacer on the lip on the inside to tighten it up.
Why it works:
Instead of the tongue being the week point, it moves it to the screw that holds the tongue and spreads it along the entire piece of metal so that when force is applied, it actually pushes out the screw into the middle of the door- the only failure point is if the metal door collapses. Most mail thieves won't spend the time to make it collapse, too much effort.
Step 5: The Guard
Using the "L" shaped aluminum bracket, cut it to size (width of box opening) and you may need to trim down the edge (lengthwise) to allow the larger tongue to pass under it once it is installed. I had to trim it down to 5/8" and carve out a space for a lock (see picture). Test fit the guard and trim until it fits so you have room to close the door AND the hatch above can also open enough to put in mail AND there is room to drill for the rivets. If you can't do all three, be patient and keep working at it. Once you find the sweet spot, mark your holes and drill the the guard. Make sure the holes in the guard are far enough away from the edge that you can get the rivet tool in.
Then, clamp it to the box and using the new holes, drill the rivet holes and attach the rivets. If you did it right, it should look like the last picture. This guard will also tighten up the door with the modified tongue.
Pros and cons on this modification: It should help keep the mail secure, but if they really really want to get in- they will need to destroy your mailbox to get there. So a $5 fix to fix the latch after they break it vs a $120 mailbox. I will let you know what damage occurs when they try again.