Disc Wall

You will run into this one before you ever craft it, since floating islands are built out of it. Break the back wall of a sky house and you keep the pieces, which is usually how a first stack gets started.

It counts as a safe wall, so a room backed with it will not spawn enemies behind you. That matters more than the look. Natural cave walls do not have that property, so replacing them is half the work of making any underground base livable, and this is one of the options that does the job while still looking like it belongs up in the sky.

Shimmer turns it into Crescent Wall, which is the easy way to get the other sky pattern without going hunting for it.