This is a mod of the original belt tensioner of the Prusa i3 Hephestos, but should work with all prusas.
Instead of the 623ZZ of the Hephestos, I used a 606ZZ bearing.
The bearing uses a 6mm bolt, 2 small washers and 2 big washers and 1 nut
The screw in the back in order to provide tension is a M5, any longer than 20mm will work fine.
The axis fixer screws are m3 x 15 without washers.
Easy to assemble, no need to disassemble the entire axis to fix it.
Update: Solidworks 2013 files included.
For bigger strenght print with solid infill and 5 degrees higher than normal prints.
Brim settings are recommended.