Why not download the free sample skybox from my site, it has boxes in various sizes and formats, as in a box with nornals reversed or a box from 6 flat sheets, the UV mapping is on all the boxes and they are in different sizes and model formats, then just swap the textures of mine to yours, you can either do that in a 3d prog or just edit the .x, just change to .txt, edit the textures for yours, then change back to .x format.
Use Bryce for the skybox to make the textures, or just use blender to make the 6 images, then put them on my model.