I installed a portable AC unit in a sliding patio door by purchasing a used pet door insert that ran floor to top of the opening. The "doggie door" was at the bottom. I removed the door and fabricated a galvanized sheet metal piece with a hole in the middle the size of the exhaust fitting from the AC unit. A little silver tape (not duct tape) and the unit was air tight, functional, and still let light in the room.
That is probably more than you want to tackle. You would have to cut a lot of the pet door insert off. Your best bet is to check with the manufacturer of the unit you want to buy. They face this problem all the time . . . and will have a solution for you.