I created a 10.11 "basic install" on a USB flash drive, with a "bare bones" account on it (so I can boot and log into the finder).
It has only the Apple "basic install" apps on it.
The only third-party app is CarbonCopyCloner so that I can boot from it, and then it will "replicate itself" to another drive.
It takes up about 11.7gb of space.