I’ve tried doing it with a sphere cut up, but I couldn’t get it to match up properly, so tossed the idea for now. Also tried an arch, then using the vertex tool to try and align it. Got it to work for the first few pieces, but once I got to the center it all went nuts on me, so I dropped that idea. How would I go about forming the corner? Forgive me if its an obvious answer, found nothing in any tutorials, and its beyond my limited hammer knowledge.
Create a sphere 2 times as large as the corner you need to fill, clip it into an eighth then put it into the corner. I would personally suggest making the brushes that form the curved edge have no face that is the same angle as the one it is connecting to. Make them only the bit that curves. That will make the sphere easier.