The c-clamps will grip over an 8-10" area. This plug will serve no purpose and may actually make the install more difficult.
If you install the non-slip tread strips on the wall of the mount before installing and buy a 3rd c-clamp for the mount, you should be able to minimize the amount of torque on the clamps and prevent crushing the pipe. The pipe that you used has a much thicker wall than regular masts, so this will also help prevent crushing.
If you install the non-slip tread strips on the wall of the mount before installing and buy a 3rd c-clamp for the mount, you should be able to minimize the amount of torque on the clamps and prevent crushing the pipe. The pipe that you used has a much thicker wall than regular masts, so this will also help prevent crushing.