I just finished drywalling a room. One of my sheets that goes up against the ceiling has a gap. I'm guessing the ceiling is sloped, coz the drywall is square - one side is snug, then about midway accross a gap starts, and gets wider as it goes along - by the end of the drywall it's probably about 1/2 wide. What's the past way to securely fill this? It's too narrow and angled to cut a strip of drywall for - do i just pack some mud in there and let it dry?