He’s using a plain old piece of threaded rod along with a coupling nut (they’re usually 3/4″ long or so). The nut is held firmly on the door using a conduit mounting bracket, ...

Thread the rods into the coupling nut no more than 1/2 inch on each end, to keep the turnbuckle as long as possible. Thread one rod into the coupling nut by turning it clockwise and the other rod ...