Just noticed there are two long flat bar mountings missing, that run up inside the panniers to the rearmost big bolts on the back of the bike's frame. They are the ones that support most of the weight. I would not want to use it to tow a well laden trailer without those.
I built one for the back of my wing a couple of years ago. My tow-bar takes the old style 50mm ball hitch. All you have to do is unbolt the tow-hitch and bolt this in its place. Worked great on my Europe tour with the missus.