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".