I bought a cheap China Geeetech Prusa i3 printer, and the idlers just came with two 624ZZ ball bearings, so the belt didn't have any guidance and rubbed against the sharp metal all the time.
This pulley solves that, it is made to fit over two 624 ball bearings and guide the 6mm wide belt without destroying it.
Apparently this is compatible with the Geeetech Delta Rostock G2 and G2S.
Please note, a standard tolerance of 0.2mm is set to compensate for the overprint of the Geeetech printers. if you want to print it on another printer or it still doesn't fit you can change the tolerance in the SCAD code (In the Customizer App or in OpenSCAD directly).
Please DO NOT scale the .stl file, this would create wrong outer dimensions, please use the customizer.
Let me know if you have problems or successfully printed it!