How to make a prefab skybox
From The DarkMod Wiki
How to make a prefab skybox
The easiest way to use a skybox in The Dark Mod is to use the prefab ones which come in the resources.
The first thing to do is create a textured box around your level. This can be caulk or texture, the important thing is that it has a texture so that the level is sealed.
Texture the inside facing sides with the textures/textures/smf/portal_sky texture.
Go to the edit menu, then “Load prefab” and navigate to the “prefabs” folder in “darkmod” folder and select “tdm_sky_starry1.pfb”.
Place this outside the main hollow box of your level. It is caulked on the outside and sealed so it will not leak.
Finished. When you load your level you will have an attractive night sky with clouds moon and stars.