Also see the ENDER 3 PRO X Belt tensioner at https://www.thingiverse.com/thing:3319649
This thing UPDATED 1-30-19 -- upper portion was colliding with bed guide wheels.
I love the design by MakeChetti!
The main fixture body was modified, lengthened and 2 "stakes" added for the 40/40 rail under the bed of the Ender 3 Pro. The slide is 1.25mm longer and a little narrower in the cavity to fit the bearing that came stock with my Ender 3 Pro.
Also the adjustment nut was remade with the same 24mm / 2mm pitch and clearanced .2mm - it was too tight on my print. This nut will work well with the original. It is a little larger in diameter and only 12 dimples.
STL Files and STEP files included in case you need to modify it - the threads should be ok with the added clearance , if not stretch or shrink the X-and Y axis on your slicer
I used a 25mm x 4mm screw for the bearing with the stock nylock nut, but using MakeChetti's original "bearing_bushing.stl" you could make the stock, longer bolt work.
Good Luck!