NOTE: Tension Bolt Side-print, Reinforced Tensioner case, and Easier Tensioning cap are part of one set; Tensioner Case, Tension bolt, and Tension Cap are part of a second set for good fitting. If you want to tighten the belt to a very high tension, I recommend the first set to make it easier and less likely to fail.

Update - 10/20/19: Adjusted Tensioner Case and Cap. One plug of the case may collide with a screw so you should snip it shorter 2mm with clippers to make it fit snug. Cap for the side print bolt is tighter to prevent thread slippage.
Update - 10/10/19 : Afraid of the bolt breaking from layer separation? There's now a side-friendly print version of the Tension Bolt and the Tension Case to accompany it!

Ender 3 or Pro X-Axis Belt Tensioner for short belts. Found the original version was too tight for my belt and remixed this one for the tighter belt that my 3D printer has.

It only tightens the belt up to an additional ~10mm but my belt is already somewhat tight on installation alone.

I made several revisions:
-sleeker design
-shortened overall
-tensioner case allows looser initial belt position for easier installation
-tensioner bolt allows tightening of the bolt onto the bearings with no issue
-stronger bolt
-different bolt and case alignment method