3D Skybox Question

[TF2CutContent]

-snip, quitting-

 

[Zeigh]

Making the map boundaries the skybox means the sky_camera entity is in the actual map, that's why you see your map "mirror image" in the skybox. What you need to do is make a small hollow cube in some inaccessible place, put the skybox tool texture on the inside faces, sky_camera in the middle and it works. You just need to make sure the skybox is somewhat bigger than 1/16 of the map.
And set a skybox texture in map options.