Correct me if i'm wrong, but is there not a raked window that has an option to "slope to ceiling" or "slope to roof"?
I'm going by memory here, which is not terribly reliable.
I'm sure i've used it before, but just can't find it; I thought it was in the "specialty" library.
I've also tried making my own, but don't see that as an option for custom windows.
Maybe I'm just dreaming....