I usually just cold crash in the keg with the same method. Give it a couple of shots of CO2 and release to purge and seal it with another shot.
When crashing in a carboy I just used plastic wrap and a rubber band or aluminum foil. With the plastic wrap, it would get sucked in a little. With the foil I assume some air got in. I'm not sure at that stage the small amount of air would make any noticeable difference.
I have heard stories about farmers back in the day using a balloon on carboys of hard cider. When it burst, the cider was done.