Our house had no window stool on one window, just a space. I have one in progress but am unsure how to have it meet the metal base of the window, onto which the window itself rests. I don't see any other way except to butt it up against this metal frame. We'll probably have the window replaced this summer with a vinyl one if it's better to wait until the old window is out and then go under the new vinyl one I could do that too.
Since you are going to replace the window i would not go to too much trouble now. Just cut a piece of cheap 1/2" pine or MDF and silicone, maybe tack it with brads, in place so you can pull it easily when the time comes to replace the window.
You could take a level and straight edge and clean up the top of the drywall so that the new board sits nicely flat just under the metal edge of the window. Do not modify to much since you do not know how your new winder is going to fit yet.