Display on FireTV?

anybody knows a way to use the new display function with a FireTV? An app to simulate cromecast maybe.
I tried AirReceiver set in chromecast mode, but unfortunatly it didn’t work.
I don’t want to use a browser.

Let me Google that for you …
AirScreen turns your Fire TV into a Chromecast with support for Google Cast and Apple AirPlay

Thanks, but “For unlimited use of all features, you’ll have to pay $2.49 per month or $19.99 per year” ist not what I’m looking for.
Before paying half the price of a Chromecast per year, I will buy a Chromecast.
And it is not given that this app is really working with Roon. AirReceiver has similiar features and it does not show as a display in the Roon settings.
That is why I’m looking for an answer in this forum. There are a lot of apps promising chromecast support, but are not compatible with Roon. Maybe somebody had found one, that will work.