Once you have the legitimate Install macOS Sierra.app (approx 4.8GB), you can manually create your own DMG for backup:
If you have a Mac that is currently running a newer OS and you want to downgrade, the official route is surprisingly difficult. Apple does not openly host Sierra on the App Store for new downloads anymore. Download Mac Os Sierra 10.12 Dmg
This will "install" the installer app into your Applications folder. Once you have the legitimate Install macOS Sierra