I wanted to put a tonneau cover on my truck and I needed a tailgate lock. This solution works really well.
First I changed the handle to an all metal one. I used a piece of 1" aluminum angle for the frame of the locking device. The housing for the plunger is plastic to avoid corrosion or rust, and the plunger is brass for the same reason. Drill the hole in your tailgate and when you install the cam lock it attaches everything to the tailgate.
Once the plunger is between the handle and the sheet metal on the tailgate the handle will not open the tailgate. The tailgate can be locked or left unlocked by the simple turn of a key.
I also put a matching lock, keyed alike, on my gas door.

 

Very nice

 

I went the other route and had a power tailgait lock installed

 

That is a neat idea. I want to eventually get low profile hard tonnue cover for my Ranger. I would need a tailgate lock too. Do they have the lock available in black instead of chrome? That handle looks like one for a 87-96 F150.