Want to disable built-in screen by default

xrandr is a difficult way to perform this; I don't recommend it.

Go to System Settings > Screen Display.

You will see both of your screens.

By selecting a screen, you can modify its defaults such as its resolution — and you can turn it off altogether (slide the ON button to OFF).

Be sure to locate the Launcher on "All displays" so that you don't end up missing the launcher.

The settings should be remembered after rebooting.

Not what you're looking for exactly, but remember you can always access your computer even without any local screen - as long as you have networking configured so that it is ready upon starting. When you install sshd on the computer you can always log in via ssh from a second machine, and reconfigure your X.