or you could try building it with brushes in a seperate "box" and use the _skybox entity.
get the outlines of the map (there where every bit of your sky sits) and do 5 "lines" of the small 3d grid in every direction.
the trick is, it doesn't work with big amounts of sky - that'd destroy the illusion.
so - jump maps or a very special frag map with small amounts of sky.