From Valve Developer Community
Finding the model
Some models available in CS:S are:
Placing the model
- Make a 3D skybox.
- Place the model in the map as a prop_dynamic or func_brush if its brushwork.
- Put the prop_dynamic by the center of your sky_camera in the skybox.
- Be sure to make it so the entities Shadow Property is set to Off else it might block the sunlight in your skybox.