This is a modification of the original R4 extruder carriage for Prusa MK3s / MK2.5s to make it compatible with pekcitron's Bear X-axis, as the MK3s STILL does not offer a decent X-axis belt tensioning mechanism.

The original Prusa carriage was modified in Solidworks based on the sources by jzkmath.
The belt path is modified to match that of the Bear Extruder but is otherwise completely compatible with the MK3s extruder.

The belt chanel was tightened w.r.t. the original prusa model to prevent the belt from slipping. In addition the belt channel is extended beyond their default length to accomodate longer belts without the need for trimming them.

The back part of the carriage was also modified in order to maintain belt access in its relocated position.

Update [17/4/19]:
My MK3s updgrade parts have arived and I have now been able to assemble and verify the workings of my Bear X-Axis - MK3s extruder mod. To my findings it works perfectly!
There was one minor discovery during the upgrade process: Going from the stock MK3 X-axis to the Bear one the required belt length slightly decreases. Though accounted for by the lengthened channel of the X-carriage, I have updated the carriage with some additional space for storing the ends of the belt so you won't have to trim them at all.

Update [27/4/19]:
As I was using this carriage with Igus RJ4JP bushings I found the ammount of play on the extruder carriage due to Prusa's oversized cutouts unacceptable. Therefore I have made a remix of this design which snugly fit around the RJ4JP bushings I'm using.
If you are using these RJ4JP bushings instread of linear ball bearings I would advise you to look for the remixed version.