Use a double mattress cover for a double mattress or a double boxspring.
Double/Full Mattress Cover
Mattress covers are essential for moving or storing your mattress, box spring and sofa cushions. Made of heavy-duty plastic material, these mattress covers will easily protect your mattress against dust, dirt and water. They slip on and off easily and only need some tape to seal the opening.
Remember to order additional bags for matching boxsprings.