Much as stated on the original from Archangel - I've had a couple of MX Master mice and they've all failed this way - the rubber around the the thumb button fails and the switch ends up continuously pressed. 

This means that the cursor doesn't move/work in any way.

I printed Archangel's and liked it - so I sat down and tweaked some of the tolerances - this sits a LOT closer to the edges and conforms a lot closer to the original profile of the mouse.

Look at the shot from the heel of the mouse as a comparison./

Shown here printed in PLA Matte black which does the best job of hiding.

Build instructions - I followed the instructions on the original - [https://makerworld.com/en/models/1011870-mx-master-thumb-button-replacement](https://makerworld.com/en/models/1011870-mx-master-thumb-button-replacement)

Solidworks part available as well