I used a section of the same mast. Remove the exterior track and then cut the flat section out or off. Narrow the flat section until it it will just fit against the inside of the flat section of the mast. Remove material until the main section and the flat section have a real tight fit on the inside of the mast and the two parts have a tight fit against each other. Now you have your splice.

The main part is springy, it pushes against the inside of the mast. If you fit it right the main part presses the flat part against the inside of the mast.I figured I had it right when it took light taps from a rubber mallet to get the spice inside the mast.

The splice should be 3 mast major diameters inside each side. The splice should be 6 mast major diameters long plus any damaged section you want to replace