Okay.
Skybox brushes don't hide stuff that's behind them. You can't use them to make concealed spaces to give the illusion of entering somewhere that's much larger than its outward appearance, like the TARDIS in Doctor Who. If a player can walk to the place that's being hidden by Skybox brushes, they will render that place through the Skybox brushes. If you want to hide something you have to cover it up.
I don't understand why you have a spawn door leading to what is essentially a void.