Sourcing a skybox

I've recently been working on a map and have just discovered the skybox I was using was not a stock Xonotic one but was instead tied to a particular map (dissocia) - I don't know the license status of that skybox so I don't know if I would be allowed to package it with my map. I also can't find more details on that map that might resolve this.
So, I'm looking to source or create a new skybox. The needs are simple enough - dark stormy clouds. Anyone know any good candidates or any good tutorials that might help me make one myself?

Maybe this will help you create your own Skybox :
1 Space Scape
  http://alexcpeterson.com/spacescape/
  https://www.youtube.com/watch?v=cC7XSL-R9tM (Space Scape Creating A Universe)
2 Gimp
  https://www.youtube.com/watch?v=P29p8-0K1Yg (BGE, Gimp - Making a skybox from an image)
3 Photoshop
  https://www.youtube.com/watch?v=YeURSgyZdGM  (HOW TO PAINT CLOUDS in photoshop)
  https://www.youtube.com/watch?v=T-U74BhB3Oc (Create Custom Skybox for Unity with Photoshop)
  https://www.youtube.com/watch?v=-ZutidNYVRE (How To Make a Unity Skybox with Photoshop)
  https://www.youtube.com/watch?v=aiU1JXz9ZAM (How to Paint Nebula Skybox, Photoshop Blender Unity3D [timelapse])
  https://www.youtube.com/watch?v=NxyH7Spb4Ls (Tutorial: Creating a Nebula/Space scene in Photoshop by Qehzy)
4 Blender
   https://www.youtube.com/watch?v=MSii7P0j420 (Blender Series: Create A Video Game Skybox)
   https://www.youtube.com/watch?v=hJd5WS6e9Cg (How to Make Realistic Clouds in Blender (100% PROCEDURAL!))
   https://www.youtube.com/watch?v=TJ_OcBh7F5I (Clouds Tutorial for Blender Cycles)
5 How to Paint Clouds?
  https://www.youtube.com/watch?v=GqgA3XyC5mg

Hipshot skyboxes are there:

https://www.quake3world.com/forum/viewtopic.php?t=9242

Oh, I have found it:

https://wwwtyro.github.io/space-3d/#anim...&sun=false

Skyboxes generated by that tool are acceptable to use. The tool itself is licensed under the unlicense. Source.

Seems that it's amazing how skybox can be created inside game. The tutorials from DarkFox are good!