The latest two hangers I made were 3cm wide which I liked much better than the 4cm version I have. 2.5 cm would probably still work too but narrow hangers can get uncomfortable with heavy weight. The sweet spot is probably somewhere around 2.5-3cm imo. When you use toothbrushes or pencils the sides will dig in further than you want and you can easily get bruishes when moving up in weight to about 4-5Kg.
PVC is toxic? I made several hangers using PVC and gained with these but the problem with PVC is that it’s not rigid enough. It will bend around your penis when tightening. Rigid sides will give the best grip, the better your grip the safer the hanger. You can cushion the sides if you want to with a thin layer of rubber or perhaps leather (haven’t tried that yet), too much cushioning will cost grip. The surface of the sides should remain smooth for optimal comfort.
Hanging whether you’re using a noose (very dangerous) or a decent hanger is effective as long you’re patient and have the time to put into a hanging routine.