Radiating arched top window niche

[JohnDepner]

I have windows that I would like to depict in Chief that I can't figure out. I'm starting with a rectangular window and the niche radiates out in an arched fashion. I'm attaching a photo showing one we built with a sill. The ones I would like to draw have no sill, and the angled sides extend to the floor creating a niche where you can stand. The wall has been thickened an additional 8" to accommodate the niche.