The lack of nodrawing isn't the problem here. If something is outside the map, it automatically won't be drawn. The problem is that the underside is not outside the map, aka the skybox is a hollow cube
edit: psy beat me to it
Y'know, there is a little trick one could use in that situation which is texture replace any remaining dev textures (within a cordon of course), with nodraw. Which will have the knockon effect of nodrawing any other faces you forgot.
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