Idler Sprockets

[Lil' Lavery]

You could use 1/2" PVC, since it has to free spin anyway. The OD would be substantially less than the sprocket (~2"), but that may or may not matter depending on your design.

Delrin could also work. But with both PVC and delrin, you have to worry about the chain wearing through the material over time. Even "grooves" in the material start to add additional friction to your device. So the better it spins around the shaft and the easier it is to replace as parts get worn down, the better.