You could also build the bottom of the entire window itself down to cover the exposed area. The build down would follow the exact same curve as the window and would meld in with the look of the window.
I had to look quite a while, but I found a photo of a bow I did about 1990 where I did it that way. I posted the photo on the first line, the middle photo on my photo page of my website:
What I remember most about that job was that I had a horrible time doing the bottom because of the overgrown prickly evergreen bushes that were shoved right into where the bottom of the window was. It was not only hard to see what I was doing, but I was a bloody mess by the time I got done. I came back the next morning to work on the other windows and the dirty rotten homeowner had cut all the bushes down. I was pretty steamed! Like he couldn't have cut them down before
I cut and scratched myself half to death.