How do I set an actual texture for a 2D skybox, rather than the default one? When I set it to a different texture in map properties, it just shows the void in-game.
That's because, for whatever reason, skyboxes are not six-sided cubemap style textures but a set of six distinct files each with its own material file, fetched by appending "lt", "rt", "ft", "bk", "up", and "dn" to the name of the skybox you specified. You probably just entered one of the actual textures with the suffix included.
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