If you want the darker part a little thicker, fleece could work. Keep in mind though that some fleece curls a bit after cutting, so you might want to finish the edges before sewing them together for a flat seam. If not, then I suggest flannel for all of it because it is thinner.
Good, Cheap, or Fast
Pick any two...
If its Good and Cheap, it wont be Fast
If its Fast and Cheap, it wont be Good
If its Fast and Good it wont be Cheap