Can you suggest me a way to make the skybox?

So I am making my first map and so far it looks like this:
http://i43.tinypic.com/14vgplc_th.jpg

I’ve read that I shouldn’t just make a hollow cube around the map unless it is small. I tried making the skyboxes like this, but I still can’t see the top parts of the wall in-game. To see them I have to put the skyboxes further away, but I don’t want people to be able to get on top of the wall.

You can make a box around the level out of 6 brushes and use the playerclip texture to block people from climbing on top of the wall.

Does the skybox have to completely enclose the area or just the places the players are able to see? I used the old ceiling as playerclip, made a new one a bit higher up for the skybox and added another 4 skybox brushes for each side. It still seems to work.

there’s nothing wrong with putting a hollow skybox-textured cube around the map. just make sure that it fits snugly around the map. that being said, if the map is exceptionally large, fill in the empty areas with func_visclusters so it doesn’t take too long to compile