I was originally going for the 32 inch, but I made a cardboard mock-up and it obviously was too big. Then I cut it down for the 28 inch size and it worked.
That piece of foam at the bottom gives it a little support and vibration damping but also lets it slide easily. I have the mount tightened to where I can adjust it but it won't move by itself while under way. The distance of the arm of the mount is about 5 or 6 inches between the hinge bolts.
The mount is screwed on the backer board from the rear and then mounted on the wall. I spent lotsa time getting the measurements exactly as I wanted.