I took an older 15 degree sierra design down bag (3 lbs) and had my amazing wife modify it so that my gathered end hammock (grand trunk UL) would slide through. The bag functions like a peapod. It works great.
I was worried about cutting into a down bag, but she made the incisions by first sewing around where the cut was going to be made (like a giant button hole). So then when we cut the fabric no down escaped. And then she lined the hole with grosgrain. The shoulder slot sits between my shoulders and my head. That location causes the hole to get taunt when I'm in the hammock and closes up so there is no draft or cold spot.
This is a very easy to set-up, comfortable, warm setup. I'm 5'10 180 lbs with broad shoulders and it fits me fine. It's not ultralight but it's light. The bag still functions fine on the ground. May add some velcro to the slots to keep them closed when using on the ground.
This was my first quick hang for this photo. For a more comfortable lay, I'd hang the foot end a bit higher and have the sleeping bag further up the hammock.