Anything thin and hard (like a cardboard) would do it. Just put it against the opening (like the door would be if it were closed), leave a small gap on top, poor from there.
When you are done, close the door while the cardboard is still there, the more you close the door, the more you slide the cardboard away (you want the door to always be as possibly closed as possible, so touching the cardboard).
Not sure I made myself clear, and haven't tried it, but pretty sure taht would work.
When you are done, close the door while the cardboard is still there, the more you close the door, the more you slide the cardboard away (you want the door to always be as possibly closed as possible, so touching the cardboard).
Not sure I made myself clear, and haven't tried it, but pretty sure taht would work.