How do we make grooves to retain our polycord?
We don't. We use flat belting.
We use McMaster-Carr p/n: 6075K19 flat belting on a simple PVC pipe. To control the belt tracking we put 3-4 wraps of hockey tape on the center of where we want the belt to be. If the belt gets pushed, it tracks back on to the tape auto-magically. The belting is a bit expensive, at $4/foot for the 2" wide stuff we use, but it requires fewer lineal feet than round belting needs to cover the same span. Additionally the flat belt typically has a smaller minimum roller diameter than round belt which can help packaging. Because we do not need to cut grooves in the roller, the wall thickness can be thinner thus saving weight and cost in the rollers. The big advantage is time saved by simply not needing to make parts.
Here is an example from our 2012 conveyor.
