Back in 2017, I started a small mod project for Portal 2, that involved doing tests caved out inside the moon.
It was shortly after scrapped, as I lacked the skills necassary to continue this mod, along with lack of interest and motivation.
All layout, and level design was done by me.
You can still check it out, and even download the unfinished build here: https://www.moddb.com/mods/portal-2-lunar