I would back it with fabric of a weight similar to one that is in the top...probably corduroy or wool. Flannel just seems too light for something like you describe, imho, and corduroy is just as cuddly. No batting or else use the flannel there to soften the backside of the seams making it more comfortable to curl up under. I would also tie it, maybe adding some embroidery or just use fly stitch to do the tying.