I tried your shader as well and applied it and its still nothing but a black box... Im wondering if theres something more I need to do with creating a box around my map. Do I need to just apply the shader on the inner faces, and apply something else on the outer faces?
EDIT: My bad haha. I just had fog turned on this whole time and it was disabling me from seeing anything!! So I had a skybox this entire time. Moral of this story? Don't put fog until your done with the map.