As seen in this video, many padlocks can be defeated in a minute or two. A really secure padlock can cost around $100 US, or more. We will be moving across several states in a few weeks with a rental truck. I want a reasonably secure lock on the truck without spending $100 for a lock I will likely use only once.

Most attempts at breaking into a truck parked for the night are crimes of opportunity conceived on the spur of the moment, as in, "Hold my beer and watch this!"

I finally chose the Master Contractor lock shown in the photo for several reasons.

It has a long enough shackle (the "U"-shaped working part of the padlock) to fit through the handle on the back of the truck.
It has two notches in the shackle for internal bearing balls to prevent opening the lock with a shim from a soft drink can.
It has a boron hardened shackle 7/16" in diameter (11mm) that can be cut with a 42" (1.07 meter) bolt cutter after a lot of struggle.(See this video. The back of a truck does not offer the kind of leverage that made cutting the shackle of this lock on a concrete floor barely possible.)
It looks fierce. One customer comment on this lock said it looks formidable enough that most thieves will move on to another target.
I was able to buy the lock locally. I was better able to look it over than if I had ordered it on-line.

The weak point on these locks is access to the cylinder at the bottom of the lock. Some of these locks have a weak cover on the cylinder that can be pried off with a screwdriver. That allows the cylinder discs to fall out and the lock is defeated. The cylinder cover on this lock is fairly heavy and appears to be anchored well at both ends in a way that makes it difficult to pry out. Another weakness related to the cylinder on these locks is that they can be picked if someone has the right tools and a little skill. But, this also requires a more determined thief who has some skill and has planned ahead.

I intend to cover the body of the lock in steel that will make access to the cylinder very difficult. It will still be relatively easy for me to open the lock when I need to do so.

Step 1: Cut Steel Scrap

I was saving some 14 gauge steel. I used my centering rule to mark the steel for cutting in two equal pieces.

Materials

14 gauge steel
M5 x 12 metric screw with an Allen screw head.
M5 nylon stop nut
Trim Head Flat Washer

Tools

Vise
Weldor's chalk marker
Angle grinder with cutting wheel
3 pound hammer
Vise-Grip pliers
Drill
File
Allen wrench

Step 2: Bend Steel and Mark

I bent the end of one piece of steel to an "L" that fits around the pad lock. I marked for a second bend at the bottom of the lock.

See the second photo. The second "L" bend extends a bit beyond the lock. I trimmed the excess off later so the end of the bent steel is flush with the surface of the lock, or nearly so. I marked the steel to cut out for the shackle.

Step 3: Cut to Fit Around the Shackle

Cut for the opening to fit around the shackle. See the second photo. I used a Vise-Grip pliers to bend and break the waste material. Do the same for the other side of the shackle. I used a grinding wheel to smooth and fit the openings after breaking away the scrap portion.

Step 4: Possible Problem

Bending steel is not always as precise as I would like. It is possible to tip the lock to one side so the cylinder is accessible. But, there is a fix for that.

See the second photo. I am bending a second piece of steel to fit inside the first piece. The first piece of steel is already in position on the lock. Notice the chalk mark on the second piece of steel. It will fit inside the first piece of steel and will eliminate too much looseness that might give access to the lock cylinder. Cut openings for the shackle. See the third photo. The two halves of the clamshell will look like this when finished. See the fourth photo. Both halves of the clamshell cover fit around the lock and it cannot be tipped to the side for access to the cylinder.

Step 5: A Screw to Hold the Bottom Halves Together

The bottom of the clamshell cover is held together with a screw over the cylinder. I live in the USA where metric screws, bolts, and nuts are available; but, they are not prevalent. I chose a metric screw with an Allen head. Most thieves will not likely arrive equipped with metric Allen wrenches. See the second photo. A special wrench I made is thin enough and the right size to fit the metric nylon stop nut. See the third photo. The screw and stop nut are not tight in use, but spin freely. The purpose is to cause even more difficulty for a would-be thief. That is why the special wrench is needed to remove the nut from the screw. To slow down anyone who might want to cut the screw, I used a trim head flat washer. It, also, is not tight and will spin when a cutting tool is applied.

I fitted half of the clamshell cover over the lock and marked so I could drill for the screw. After drilling, I fitted the other half of the clamshell and marked it for drilling.

Step 6: To Open the Lock

In order to open the lock, slip the special thin wrench in from one side and turn the M5 screw until the nut releases. Wiggle and shake the nut out from the side. Do not lose it. See the second photo. The clamshell fits very firmly. Pry the bottom of the clamshell apart enough to fit the key in the cylinder using the special wrench and a screwdriver. See the third photo. Insert the key and turn it to open the lock. (The screwdriver, wrench, and Allen wrench will not be kept in the truck, but in our luggage inside the motel we use.)

To lock the lock, insert the M5 screw through the washer and the holes. Tighten the nylon stop nut on the screw only until the end of the screw is flush with the end of the nut. (I did grind the nut a little thinner during fitting.) Slip the opened padlock into one side of the clamshell. After slipping the open end of the shackle through the handle holes on the truck, turn the shackle a half turn and close the lock.

Fitting the two halves of my clamshell protector around the lock so I can still open the lock fairly easily, as well as position and close it, took more time than I expected. Yet, someone with enough tools, patience, opportunity, and time to remain undetected will be able to get into our truck eventually. But, my clamshell lock cover and very hard shackle should cause any potential thieves enough second thoughts that they choose to look for another target. And, I will park the truck at night so the rear door of the truck box is facing into the main part of the parking lot where street lights shine on it, not away from it where a thief can hide in the dark while at work.

Step 7: Update

This is the lock on our rental truck. The door latch is a Whiting. Todco latches are nearly identical and are used on many trucks. I added short pieces of black pipe to make getting a bolt cutter onto the shackle more difficult. If it was effective the appearance was probably more helpful than anything. But, I saw no evidence anyone tried to compromise our lock. I did need to open the truck door during the trip to load a piece of furniture my brother was keeping for our daughter. We traveled 2700 miles and parked the truck overnight eight nights.