This Aluminum GT2 Timing Idler Pulley For 6mm Belt 20 Tooth 5mm Bore pulley works with GT2 6mm wide timing belt; it will work perfectly. This idler is a good match for 20T GT2 driving Timing Belt Pulley.
Many DIY 3D printer makers are using idler pulleys based on 3D Printed wheels. Inaccurate printed idler decrease printing quality and create vibrations since the outside diameter of the idler is not concentric to the rotation axis. Other makers are using double bearings with stationary washers clamped from both sides.