Just to let you ladies know. That the sleep sack or Sleeping bag liner is available at most sporting goods. It is used to line a sleeping bag to retain the persons heat in colder weather.
They can be made by sewing the bottom and part way up the open edge side of your chosen fleece fabric.
Hope that this helps with any questions you may have. Although you have had many great responses before my contribution.
Merry Christmas and God bless.