I've seen quite a few maps with skyboxes, and I don't think anyone should even be close to releasing a map with a skybox, so I just whipped this up in about a minute. This is an example map for early mappers who haven't learned much. This just basically teaches what to do and what not to do with skies. It shows the the BAD skyboxed, the BETTER sky-ceiling, and the ALSO BETTER extended sky-ceiling to fit the rocket.
UPDATE: Included a .TXT file in there informing about pros and cons about the 3 different sky methods.
Although I must admit I've tested Skyboxing vs. not a little, and I haven't seen much or in some cases any, in the way of performance increases--at least with my maps.
It might be helpful to include a text file listing the reasons why skyboxing is bad, besides just the blanket statement that I hear all the time, that; 'The engine was not made for it.'
If you throw a granade over the wall, it comes back cuz it bounced on the sky.
But if you make a box, you can throw the granade where ever you want to throw it..