The current x-motor holder for the MK3 is much better for mounting the x-axis belt. It has a belt tensioner included in the design. This is not so on the MK2 files they have. Because the parts are not interchangeable I decided to fix this myself for my printer when it is time to install the mk2.5s upgrade anyway. All I did was to cut of the motor mount part of the mk3 and the mk2 parts and glue the motor mount of the mk3 to the leftovers from the mk2 part.
As the mk3 is built almost the same way as the mk2 this is a perfect fit.
More pictures will be uploaded as soon as it is mounted on the printer (when the upgrade kit arrives).
All credit goes to Prusa for designing the parts in the first place and making them open source!