It's quite simple. For the mount on the Tern bike shown in the image, get the Tern Luggage Truss. I've used it, version I and II, with the Kanga Rack around town and touring overseas, on and off road over nearly a decade, and it's solid, reliable. Ignore the chatter about the Brompton type mount (yes, I've used that one too) cuz it's irrelevant. You've got a Dahon-, not a Brompton-type mount on the head tube.