We've done a couple of different things. For short trips we've just had the kids use sleeping bags (double size) on both top and bottom bunks. For longer trips, my wife has pinned a queen sheet to a 2 inch foam mattress topper, along with the appropriate top sheet, blanket, etc. It works well on the lower bunk. That way it is easily rolled up and placed on the top bunk as a single piece during the day and back again at night. You can do the same for the top bunk.