Update 7/19/18 (finally!) - I finally got around to updating this post after a few members requested that I add in end stops to this motor mount so that is reflected in the new update (rev 3 file). I also added in a slot which is used to tighten up the set screw on the pulley. I have kept the original rev 2 file in case anyone wants that version instead of this new one.
Update 3/10/18 - I checked the motor mount in a 3D model of the prusa i3 mk3 with r2 components by jzkmath. The motor mount still works beautifully and still corrects any misalignment that is inherent in the stock mount.
I read a lot of comments from people asking if a third screw was able to be added to the motor mount so I decided to go back to the model and investigate this. I fount that I was able to do so easily by adjusting some bolt lengths and tweaking some features on the original motor mount.
I originally didn't want to reprint the motor mount due to the .5mm offset that is standard on the printer since this is a minimal error and is probably negligible. However since I had to make the mount more rigid as well as add the third bolt, I figured I would correct this misalignment in the process.
This mount is intended to be used with the stock belt holder only. With this mount there is no longer any misalignment of any type. I also added 1.5mm to the face of the mount so that the motor was moved to the side. This places the motor shaft in the proper location so that the pulley is centered on the shaft. This gives an even amount of side to side adjustment of the pulley location. so there no more issues with belt rubbing due to a lack of pulley adjustment. This also pairs nicely to the adjustable belt tensioner that I designed (https://www.thingiverse.com/thing:2786671) and with this combination along with the stock belt holder, there is perfect belt parallelism as well as no belt misalignment.
Unfortunately in adding the third screw, some hardware must be replaced from the stock setup. You will need two m3 x 30mm socket head cap screws and one m3 x 16mm (longer wont hurt) socket head cap screw. All the other hardware remains the same. See the assembly drawing for screw locations.
Thank you again to jweaver for testing as well as the pictures!!!
I am using Raptor PLA from Makergeeks. You can use whatever print settings you are comfortable with for printing in the material you choose. Print the part in the orientation as show in the preview above.