Did you know you can paint your own skyboxes in Open Brush, export them, then using them in Unity Projects or even your own paintings? Paint your scene as usual and generate a 360 photo. Once you do that, convert your photo to equirectangular (or keep it as is for stereoscopic and just upload it in your media folder under skyboxes. It's that easy. Check out my other tutorial on how to create 360 photos from TiltBrush or Open Brush for detailed instruction on that process.