My mum used to use these in the sixties when we went camping.  You got it .  She learned this trick from her mum.  And relearned it when she was a Girl Guide just after the second world war.  How did you guess.  Yes her mum was a Girl Guide Leader too.

Linen inner sheet

A sheet inside your sleeping bag will add to your warmth and keep the inside of your sleeping bag clean. Sew an old flannelette sheet into a rectangular bag shape - leave the sides open near the top. A warmer, lighter sheet bag can be made from pure silk lining material. Buy double your length and stitch the sides.