Make the bottom brush that actually seals the map area a normal skybrush. Then make another brush that's about as wide as the skybrush, but place it so it's top surface is just below the top surface of the skybrush.
What I did in Chapter's "He Falls Like Lucifer" was to make the skybrushes 64 thick, then make the "black" textured brush 32 thick, and place it inside the bottom skybrush with a gap of 16 all the way around in all three axes.
Light the top of the black brush with spotlights that are set to have a cone less than 180 degrees ( this is so the light they emit only hits the top of the black brush, and doesn't seep up the sides of the actual architecture ).
Overscale the black texture on the inner brush by a lot, and do the same to all the sky texture too.