Just FYI - it is much, much easier to file a flat on a threaded steerer than you might think (If you are threadless this would not apply). Much easier than trying to file a notch or round out of the flat inside the hanger. Rene Herse did not have the grooved style hangers, but did have the flat style hangers, so I bought one and filed the threaded, grooved steerer on my Riv custom. Took less than five minutes. Having the groove already in the steerer made it very easy to center the flat and the threads meant that you only have to file the pointed top of the threads off, the thicker bottom of the threads remained undisturbed. I only filed the minimum so the hanger and spacers barely fit on - a nice tight fit. I might make this upgrade on several bikes now.
I originally installed a Paul hanger, which is a nice quality piece, but kind of brutalist in the way that many Paul components are, and it was asymmetric due to the clamp (clamping to threaded steerers is no problem). The esthetics of the Rene Herse hanger are so much better, and I have Rene Herse centerpulls anyway.