Is there any foundation or stilts for square prefabs rooms or rooms made from parts?

I'm new to building and can't figure out how to get struts that extend down to the ground. The base is over a hilltop, so I want it on stilts. I bought a ton of parts on the anomaly, but none seem to work.

I see the foundation for round prefab rooms, but not square. And, for a room built out of the separate alloy parts, I'm not sure what to use if I put down floor tiles.

Any names of parts I should go buy? Or am I missing how this works?