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) and then once we cut the slit she lined the hole with grosgrain. So then when we cut the fabric no down escaped. The shoulder slit 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 set up. I'm 5'10 180 lbs with broad shoulders and it fits me fine.