Here is the Delta Serpentis skybox I made:Delta Serpentis Skybox
Below is a zip file containing the bitmaps I used to make it:Delta Serpentis Bitmaps
My goal in designing these bitmaps was to be able to combine them and give the impression that the skybox nebula was a contiguous nebula as opposed to a bunch of bitmaps. Each bitmaps is a .png at 3072x3072 resolution, and each one has an alpha channel. For a single nebula, there are three bitmap components. I combined these in blender. Having each light rendered as a separate bitmap allowed me control over each nebula's appearance as I combined them, which is why I did it this way.
In order to combine them, each bitmaps has a separate material that looks like this.
This material is applied to a basic plane object in blender. I used the constraint "damped track" to track the plane to an empty located at the origin. I then used the subdivision modifier set to "simple" to subdivide the plane at 4 subdivisions. Next I used the "cast" modifier with the target set to the empty. This made each plane spherical with it's center where the empty is. This allows me to make sure that the bitmaps don't intersect and are always facing the camera at the origin.
Here's a couple of screenshots of the skybox in game: