I have an '18 C. I believed the salesman when he told us those plastic roof rails were actually roof racks, and could support a crossbar. That would have been a deal-killer had I learned that before actually trying to purchase said roof rails.
So I looked around for something that would work anyway, and never decided on a kit like thule or something since it would be an expensive trial. But we needed space for camping gear, and bought this because it was cheap:
What I learned along the way is that those things that attach to the inside rail of your door work pretty well. If I had a roof rail system that attached the same way, I would buy it.
Let us know what you land on!