If you have only one 625 bearing for each Idler, this might be the solution for you.
Running the GT2 belt directly on the 625 bearing can be noisy and frustrating. So this a two-piece belt guide for your bearing.
Simply print it at your standart settings and try it out on your reprap.
It's no big deal to print this piece. Make sure that everything look great on your slicer program and print it.
The measurement of the assembled piece:
External diameter (the guide wall): 21mm
Diameter for the belt: 17.2mm
The 625zz bearing stays tight in place.