Do a search in the super duty forum. There is a way to fit a hose clamp around the right side hinge that makes it less easy to remove. It still can be stolen but it becomes more difficult.
Also welcome to FTE! I would run a search for you but im on my phone. Search functions on the blue bar at the top. Do an advanced search and choose a few forums. Tailgate lock or stolen tailgate should be your keywords.
Stainless steel worm screw "hose clamp" wrapped around the right side "cup" that serves as a hinge, it effectively blocks that "open slot" portion. Just place it so that it's not so easy to quickly locate the screw. It slows you down a little when you want to remove the T-gate as well, but thieves hate slow downs.
You can even tighten it, then cut some excess off and roll up the end and crimp it to vastly slow down the removal of the clamp even more.
I also keep my T-gate lock and Undercover locks "locked".