Guys, I'd just like to let you know Roblox has a map called Slate and it has the exact same skybox as Retails, I don't think it'd hurt if you had a similar one.
Probably a stock skybox they both dredged up somewhere.
Skyboxes actually are pretty easy. Lots of preview renders in Terragen + a few hours for super high quality final renders and you're done. Or if you're confident in your skills you can make one in the GIMP pretty easily by painting on the net of the cube, and then chopping it up, although that requires a little more blending of the edges.
Terragen is just yuck sometimes, and cannot do space skyboxes.