Until v5.6 is out, at least, we have no means to make real skybox in Alternativa3D. We can fake it by placing very large static box around our scene. The problem with this solution is that there are many ways to unwrap the box and, correspondingly, there are many incompatible skybox textures. Changing box UVs by hand every time you replace skybox texture in your project is getting boring at some point, and when they want you to support multiple textures originating from all kinds of different sources, it gets nearly impossible. And then you write this thing:
What we have here is three incompatible cubic panoramas loaded with the same SkyBox class. Additionally, there is simple class loading textures in background that may come in handy to some. Check out full source, if you wish.