When adding a dual z-axis upgrade, I needed an easy tensioner. I have a series of upgrades with the goal of making them as cheap as possible. 

I designed this to go with the beating pulley linked below, but can be used with a number of others. The height is made to go with the z-axis stabilizers (also linked below) with the printed 48 tooth pulley. I designed it so it would clip in place, so mounting upside down is also possible. This puts the tension pulley 6mm above the gantry.

Bearing pulley: https://www.printables.com/model/617934-gt2-idler-pulley-using-608-bearing/files

Stabilizer (I mirrored this for the right side): https://www.printables.com/model/187674-ender-3-z-axis-bearing-stabilizer

48 tooth pulley: https://www.thingiverse.com/thing:3330894