Sky Textures in the BF2 engine use Panoramas not sky boxes. Anyways make sure your texture is 2048x512 pixles.

I'm still fiddling with making a sky texture as I want a map of mine to feature a really kick pie sky but I'm fumbling all over the place with gimp =P.

Oh and make it an SSD ^_^ that would be monolithic!

The basic concept is this: You create a Skybox, 6 sides, and split it into a 10 x 10 grid on each face of the box. You then take a planar map of the grid and put that into your texture file so you know where the grid lines are. Now in the texture file you create your star field, and erase any stars intersecting a gridline. Next you texture in your planets, nebulae, stars etc and you can basically place them anywhere but either central for a large planet or in a line across the top or bottom is best.

You then go back into 3D max and star UVW mapping. Basically look at your sky box and say to yourself "where do I want this planet?" Pick a spot on the grid, select the appropriate grid faces and map it to where the planet is on your texture. When making the texture make sure that your planet has its outermost edge of the graphic NOT crossing a grid line.

Once you've mapped all your Points of Interest (POI) you can use remaining clear untouched portions of star field to map the rest of the skybox with empty space, rotating and flipping to create completely random space. Because nothing is directly on a gridline you get no seams whatsoever and a nice high resolution skybox.

This method is not without its limits, but when you consider that the Battlefield 2142 engine supports 1 Texture to wrap around 6 individual sides of a skybox with a maximum of 2048 x 2048 resolution size, this is probably the best way to approach the tricky challenge of putting space flight into Battlefield 2142.