Thanks
First off: My problem with the clone tool was that as mentioned, camera tracks the glass, the shadow cast from the glass is warping as the glass rolls and the refractions of the glass itself distorts the line in such a way that I, if I did it correctly, got too many slight shifts and changes within the clone-mask.
Then, to your alpha matte-approach, the goal is to isolate the highlighted line or have I misunderstood?
Kind regards