Roberta, We are making bunk bed quilts in this size for a children's home. I know this size is called a Lap Quilt, but it fits the bunk mattress well without too little or too much overhang. I am so glad you asked this question, and will save all 10 answers.
Shelbie, I'm glad to see your answer about sheets for backing. Sheets have been used for many years, but somewhere along the way, someone wrote a negative article about using them, and it has become gospel, sort of like the Underground Railroad. Not all sheets are created equal, some heavier than others.