The code Joe_Gratz posted didn´t work for me either. I created my own, by changing the display_ui step by step. And it worked out well. At least with the Roon weblink (I have no experience with chromecast). Design examples to be found here.
But: It only works, if it is the display_ui on a Win10 PC, and this same PC is the one, that is used as Roon core. It does NOT work on Nucleus or ROCK, because the folder with the display_ui is not user-accessible.