I would caution about using some of the loosely woven cotton blankets.. The texture of the weave is much more apparent than some other blankets. Check your Good Will for velux blankets they make great batting and are really really warm.
I have used some wool blnakets , they are given to me by people who washed/dryed them and now they no longer fit their bed. These are the warmest quilts as the washed/dried blanket is felted wool , very dense !