I don't think it's possible thats why the default config provided doesn't change the skybox or make many additional changes. Not really looked into it.
Edit: that background is a prop_static, compiled into the map and unchangeable.
What you could try doing is spawn some smoke or maybe fog entity or something in the small world skybox thing (8072.282226 12853.265625 -987.754272) to block that model being visible.
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