I tried to set this up. Initially I forgot to switch from Catalina to Big sur. After it finished downloading Catalina, I did that and restarted macinabox but the helper script still created the Catalina VM. I managed to delete that VM after some trouble and having to run the command virsh undefine --nvram "Macinabox Catalina" to allow me circumvent the error saying that there is nvram allocated to the domain preventing me from removing the VM in the webUI (this is probably the wrong way to do this but it was the solution that popped up on google), then remove the iso files, the custom_ovmf folder, the disk image, and the macinabox appdata folder and restart macinabox with the correct settings.
After doing all that, I got Big Sur loaded and booting. however after selecting MacOS base system for the initial install the apple logo shows up and then a loading bar for a few seconds and then it reboots with the message "your computer restarted because of a problem" on the following install attempt and I am completely unable to advance any farther.
I repeated the reset steps listed above and went back to Catalina and had the same issue. (still currently installed and not working)
I have no idea if I messed up or if my server just doesn't want to work. Any assistance would be appreciated.