I was able to get one setup by creating an iso with the installer..run the following: 

hdiutil create -o /tmp/BigSur -size 17000m -volname BigSur -layout SPUD -fs HFS+J
hdiutil attach /tmp/BigSur.dmg -noverify -mountpoint /Volumes/BigSur
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/BigSur --nointeraction
hdiutil detach /volumes/Install\ macOS\ Big\ Sur (this didn't work for me but was able to force eject with finder.) 
hdiutil convert /tmp/BigSur.dmg -format UDTO -o ~/Desktop/BigSur.cdr
mv ~/Desktop/BigSur.cdr ~/Desktop/BigSur.iso