I also tried to install BigSur and was plagued with the problem of getting Catalina. I found this information on Github and it worked. https://github.com/SpaceinvaderOne/Macinabox/pull/39 From the Container template edit screen in the WebGUI, set "Download Method" to method2. Access the container console, modify /macinabox/unraid.sh, and restart the container. I hope this helps someone else.