Sorry I can't help, I use a quilt, but I have used my old sleeping bag to improve the heat-retaining qualities of my quilt. My wife's friend cut a rectangular area (about 4 x 3) out of the old sleeping bag, gathered the outer edge down together and then stuffed it into what is now my thicker down blanket. She's fitted 4 little loops on each corner which correspond to the little lugs on my quilt, so now my quilt, when needed, can be double layered. It needed a simple mechanism where the blanket wouldn't slide off, or move in a different direction in the night, as I turn over from one hip to the other.
Until now I have fitted inside my quilt but I became too hot, when I next use it I'm going to secure it to the outside, just easier to throw off if I need to.
If I ever acquire another old sleeping bag I might even get my wife's friend to make me another blanket, just a small short and narrow one, with elastic straps at each corner to keep it secure on top of my mattress. I'm thinking about trying some winter camping and I think such a blanket might be useful?
So don't throw your old sleeping bag away, it could still improve your sleeping experience.