I created a "virgin install" of El Capitan on a USB flashdrive that can "clone itself" using CarbonCopyCloner to another drive. This allows one to "install" a fresh copy of 10.11 without actually running the installer app, going through the reboots, etc.
It takes up about 9gb+ on a 16gb USB flashdrive.
Wouldn't fit onto an 8gb flashdrive (found this out the hard way, heh).