While you’re right about the galaxy and maybe oxygen portion of this, a planet or planetoid would not be impossible or even improbable.
- Mod in two soft variants of rock, likely Dirt and Dirt With Grass
- Spawn massive asteroid, made of dirt/rock/resources with spherical gravity pulling everything at the surface and a bit beyond towards the planet.
- Bit more advanced modding incorporating a sort of soft “shell” of clouds or what have you around the asteroid, potentially functioning off a similar system to the spherical grav generator, but with no physical effect on the player
- Make shell appear blue/green/red/orange/whatever from the Planetside view, and look white/clear from the extra-planetary view.
The bonus portion on this is that you do not wind up with a flat “planet map” but a full planet in 3d space. You could even work out a method of rotation, if you really wanted to.