If they aren't too big you can get some oversized plastic garbage bags and just open them up and slip them over the tops upside down. But with some of the pictures here and the size of people's plants that might not be possible. maybe you could slip the bags over sections of the plant so they don't get soaked. if the bags are left open at the bottom the moisture won't build up inside and the ambient warmth from the ground will keep them from getting frostbite.