The cheapest option as @andybob mentioned would be the Raspberry Pi + Iqaudio/HiFIberry dac, though that would take additional setup on your end.
For plug and play solutions the SonicOrbiter SE connected via the optical output would allow you to use the receiver as a roon output.
Of course since it is a receiver providing amplification there are a ton of other options with the HDMI inputs not found on most two channel systems. A small micro PC or NUC type could be used to plug into the HDMI and you could run roon through that + at the same time have control via a bluetooth keyboard, or programmed remote. I was able to get a logitech harmony pretty function for basic roon playback using a few programs.
Honestly in your situation i think the best question is what do you want to be able to do with your receiver and Roon? Do you just want to point music at it and control Roon through a tablet/phone or do you want a visual element? Do you currently have an HTPC hooked up to the receiver?
Edit: I remembered this thread which has a device that may do what you are looking for: NetPlay Audio products special offer