There are a lot of X idlers with a tensioner mod for the Prusa i3. But why print an entirely new idler when all you really need is a tensioner that fits your existing one? This simple, concise add-on x-axis belt tensioner can be used with most existing Prusa i3 x idlers and requires very little material. If you have an unusual x-idler or another printer with a similar design, you can modify the included SCAD file to fit.

Assembly

To build this part, slice and print one of the included STL files and add the vitamins listed in the BoM. (One of the STLs prints the tensioner part on its side with built-in supports. While it requires some extra cleanup it should make a stronger and neater looking part.) Then just stick the endcap part over the hole in the x-idler and screw it into the trapped nut in the tensioner. Couldn't be simpler!

(Note that the x-idler.stl file is only included for the "assembly" view in the SCAD file.)