You just cut the bottom of the mounts off, don't take too much off, do a little at a time until it sits flat on the tower mount. The aluminum piece between is 20.2mm. You will see a couple of round mold marks in the mount, drill a hole in the center of each of those and then screw it together. Drill out the holes in the tower mount so a screw will slide through the mount, that way it will tighten properly.

One thing I didn't do yet but might if the screws pull out is to drill a hole so a small 3/16" nut can be slid in for the mount screws can have a little metal to bite into.