This is a redo of my original x-tensioner so I could replace the idler bearing with a geared bearing to reduce belt vibration. This uses the same 5mm hardware that came with my printer. This is made to use a GT2 20 teeth idler pulley with a 5mm bore. Just don't buy from BIQU! They were horrible.

Update
After using geared idlers for awhile, I really do not think they make any difference in print quality one way or another on an I3 style printer. Don't hurt, and don't help. Getting something with decent bearings and no wobble is the important thing!