When I started quilting 46-47 years ago I used some sheets for backings, much better than now.
I used a sheet backing for a comforter about 9 or so years ago and the tied comforter wants to slide off the bed easily.
I have looked at sheets and can't find any that seems suitable. I wish they made ones that could be used for backs. The size would have less excess leftovers. You could buy them closer to the size of the top.