We've made a lot of these. It needs to be a deep channel - polycord is sticky and likes walking out if you put any side load on it.
However, consider using flat polycord belting. Round polycord slips even when it's very tight. Flat belting doesn't. And with the belting, you can just build up a high spot on the roller using anything you like - tape, rubber o-rings, etc. and the belt will keep itself straight.
Disclaimer: belt can be much harder to weld than cord.
Mcmaster sells round belt pulleys.
https://www.mcmaster.com/#round-belt-pulleys/=1525gp9