Get rid of that awful plastic tax disc holder and fit one of these.
Very easy to fit using the supplied adhesive pads, ensure correct positioning first because they really do stick hard! When the time comes to fit the new tax disc just a simple twist off and on, it's done. Some cheaper versions available elsewhare use small screws to fix the cover - those are an absolute pain. This version is far better.
Looks good too.
The only criticism would be that my particular one rattles very slightly.