Since the circumference of the mast base plate is larger than the diameter of the mast, the curved shape of the inner tube works to your advantage. First split the tube by cutting along the inner and outer seam with it laid flat.
Then curl the tape measure along the inner and outer edges to your earlier measurements of the diameters. Add about 2 inches extra for overlap, and cut.
The rubber piece will now look like the section of tire in this photo.
