A smaller 30m x 30m footprint grunged, abandoned sewer room for your skybox.
This skybox is a closed skybox.
A small part of an underground sewer system with just enough room for you to live. There's about 20m x 20m of livable space but the exterior uses a little more space.
This skybox has been abused and is just a filthy, nasty place in a dark, grungey area where no outside light can get in. Luckily, it still has electricity and working lights!
And, thanks to ultra-efficient mesh only design, it's just 10 prims!