I used fleece for backing...nice and soft on the skin. FLannel would work, too. I made it big enough to cover a person from neck to a bit below the hips. Not too bulky to carry to treatments that way. Next time, i would add some kind of tie/strap so it could be folded and held together sort of sleeping bag/camping style.
Edited to add: I didn't use batting. Fleece was enough and helped keep it lightweight.