Skybox brushes are not fully opaque. The 2d skybox will always, always render behind everything else in the map- regardless of the placement of the 2d skybox brush relative to other brushes. You'll want to use some sort of opaque brush, ie the wood texture you're already using, to ensure players can't see into other parts of the map they shouldn't. You also generally shouldn't be using the skybox texture as a wall within the playable space (areas that players can access), as rockets will not explode on skybox brushes.