Alright I’m almost there! I’ve made the latlong equirectangular map from a horizontal cross cubemap, made a material out of it and applied that material to the sky sphere mesh. The only problem I have left is that the texture seems to be mirrored on the top and bottom of the inside of the sphere instead of wrapping the one texture all the way around:
Thumbnail of texture: 
I’ve tried applying a TextureCoordinate node to the UV input of the texture but giving it .5 for V, which seems like it should fix the problem, just stretches the texture but leaves it mirrored. “Un-mirroring” either axis just throws everything out of whack. Can anyone help get me the last of the way there?