This gist has been turned into a full GitHub project: https://github.com/brandonscript/Flex
hdiutil attach /Applications/Install\ macOS\ Sierra\ Public\ Beta.app/Contents/SharedSupport/InstallESD.dmg -noverify -nobrowse -mountpoint /Volumes/install_apphdiutil create -o /tmp/Sierra.cdr -size 7316m -layout SPUD -fs HFS+Jhdiutil attach /tmp/Sierra.cdr.dmg -noverify -nobrowse -mountpoint /Volumes/install_buildasr restore -source /Volumes/install_app/BaseSystem.dmg -target /Volumes/install_build -noprompt -noverify -eraserm /Volumes/OS\ X\ Base\ System/System/Installation/Packagescp -rp /Volumes/install_app/Packages /Volumes/OS\ X\ Base\ System/System/Installation/cp -rp /Volumes/install_app/BaseSystem.chunklist /Volumes/OS\ X\ Base\ System/BaseSystem.chunklist