Original model did not allow the smooth rods to go all the way in without drilling. I have added the needed 8mm of depth so that the smooth rods are exactly the same as Prusa's. Apologies for those that printed this and ran into this issue.
THIS USES A 16T TOOTHED IDLER PULLEY AND NOT THE STOCK PRUSA 20T SMOOTH IDLER PULLEY!!!! YOU WILL NEED TO MODIFY YOUR X-CARRIAGE FOR THIS AS WELL. I ALSO HAVE DESIGNED THAT PART.
Here is my newly designed X-Belt Tensioner for the X Axis on Prusa MK 2/S and I believe 3.
As you can see, the tensioner is internal to the original part so it fits exactly as stock. There is 5mm of movement so you can tighten this quite a bit. I used internal M3 hex-nuts and 20mm M3 hardware for the tensioning part.
To assemble you need to add the nuts, place the two halves together in the mount, install the pulley, and then install the nut and bolt for the pulley. From there you insert two 20mm M3's on the side and tighten until the insert is flush with the mount. That is the exact stock location. Assemble as per usual and the tighten afterward.
If you want to modify your Y-Mount for the belt as well, I have created this too.
I used PETG for this and did 40% infill for strength.