I use a Campmor 20* down bag. $109.99 and very good quality. On a 24* night, snowing/freezing rain/6-8mph wind with my JRB underquilt and the campmor bag only, I was TOASTY. I start with the bag unzipped and you stick it in the hammock first, then push it to the side when you get in. Then put your feet in the foot pouch and pull the bag over you. Use it like a quilt with a footsack or just tuck it in around you on the sides.
Works really well for me.