Not an electric idea, but when I had such an issue I built a window (glass and white 2x2s from HD) the size of the gap from the a/c to the top of the opening without the original window. Before I moved out I screwed the original window back to the mechanism.
I did the same thing and was going to give this idea too.
Mine was an awning window. I removed it from the window frame, installed the a/c using a wood cross piece to rest the top against, and then added plexi above it