The side-mount comes with an excellent ratchet strap that does a great job of securing the can so that it doesn’t move around or rattle, but it doesn’t have a provision for locking. Here’s a locking solution that can be implemented for about $7 including the lock. It’s an easy DIY project, just requires a drill and a 1/4” drill bit.
• Small u-bolt. A Stanley/National #112 is a good choice, which should be available in most hardware stores for maybe $2.00 with a zinc-plated finish. It’s also available in stainless for about $2 more. This comes with 2 nuts; for this application 4 nuts are required; they’re very common 1/4-20 nuts so most DIY’s would likely have two on hand. It’s also a good idea to use washers, one under each nut.
• Bike lock or chain lock. A Master 8152D combination lock that retails for around $5 is a good choice.
The photo below shows the u-bolt as used, with the extra nuts and washers. The strap that comes with the u-bolt can be used if you like, but is not necessary in this application.
The u-bolt gets installed in the top of the side-mount main frame as shown below. Two holes are drilled, and nuts are placed on both the outside and the inside of the frame and tightened down.
The lock cable is then threaded through the handle of the jerry can and the u-bolt.
The u-bolt is secure because the nuts are inside the main panel of the side-mount and can’t be accessed when the side-mount is installed. The side-mount itself is secure because one of the nuts attaching it to the Jeep is inside the door opening and inaccessible with the door is closed and locked; one of the other bolts is under the hood so if you’ve got a good lock that one’s inaccessible too.