I live in South Florida and only use a sheet when I sleep, so I asked around to those who keep their A/C cold enough for the Penguins. It is all according to how cold the home is kept and what she wears when she goes to bed. There is very thin batting that would probably suffice, but your idea of flannel in between would also work. Many stores, like Big Lots (if you are lucky enough to have one near you) carry a large line of Lumex blankets, many times irregulars, that you could also use and quilt. An inexpensive flannel blanket would also work. Enjoy whatever it is you wind up doing, and I am sure that the beautiful finished product will be well appreciated.
Suzanne