I've given that a go but it keeps just showing up as black when I put it on the walls. This is the skybox i'm looking to use:
http://forums.tf2maps.net/showthread.php?t=11194
What would I need to stick in the VMT to make it work? I copied one of the Brickwall VMTs and just changed the filepath but still shows as black.
Edit Nevermind I solved it. Had to adjust settings in the VTF as well!