I took a stab at it and got it working: https://github.com/jcoker85/UnraidGameServers/tree/sunkenland
All you should need to do is add the template XML manually in your dockerMan/templates-user folder (I just use the CLI and create a file with nano and then copy the contents in there), use the Add Container button in the Docker section, and select the Sunkenland template. If you have any issues just log them on Github.
Maybe they made some sort of polling API relay to do server-client communication because it works without forwarding a port, but I did notice if I forwarded 27015 I got a lot less latency with my computer locally connected to my network. When isolating it using my cellular hotspot, I didn't notice any difference though. I have no idea what's going on behind the scenes and they didn't publish any port settings (plus I ain't no network engineer), but hopefully they will publish some additional documentation as time goes on.
@ich777 Hope you don't mind me posting this here, and thanks again for all your stuff!