I would not want to try and mount any CONVENTIONAL type of latch to the top of the carrier, as the only place to put the other end of the latch would be on the back door, which is a poor candidate that would quickly fatigue from the stress.
When I was running the dual rear carriers [both plate style] on Ruftoys, I made a drawbar that pivoted off a plate mounted next to the passenger carrier upper hinge across to the driver's side carrier. I welded two pieces of angle iron [one to the top of each carrier] just far enough back from the tire itself so that the tires [and carriers]had to be pushed just slightly back in order to lower the drawbar thru the pieces of angle iron. It was not a high-tech design or solution, but it worked well.
I think you could just as easily make such a drawbar and make a small bracket on the driver's side of the rig to accept the other end of the drawbar.
Best
Mark A.