I recently released a x tensioner for use with my own x axis parts. This version is based upon the same design but with a few modifications to suit a stock x axis on the Anet A8 (The version with ABS printed x axis sides though it may work with the injection moulded parts too!)
This is oriented as intended for printing with supports built in. You will need to cut away the supports on the underside of the two prongs, the supports inside the central rectangular hole and most (not all) of the cylinder in the center.
You should ideally leave an equal amount of the central cylinder on either side such that the idler wheel is barely able to move from side to side but can rotate freely when mounted.
You will also need 3 sets of m3 screw and nut to hold the idler in place and fit to the top and bottom of the part to allow you to adjust the screws to tweak the tension in the x axis belt.