The lower opening on non A/C units are always broken from passengers big a$$ feet not sitting still and not being careful.
What I did was: I was able to get another one that was broken and fabricated a piece from it to fit the broken section. Using a pnuematic cutter tool and roloc, I shaped it to fit the curved edge. Glued it with an epoxy, sanded the joining sections smooth and painted it black on the inner opening.
It's not even noticable where I repaired it.