A prefab I made to save you the trouble of figuring out how Valve made the rd_asteroid skybox.
Just copy and paste into your map
This is a 3D skybox
Valve uses sky_goldrush_01 for the 2D skybox texture but it doesn't matter which one you use.
I put a light_enviroment entity inside the vmf with the correct lighting values
info_player_teamspawn included so you can see how it looks ingame