I use a Big Agnes Mystic, a rectangular bag. It's easy to roll over inside it and go to sleep on my stomach with plenty of room to stretch out (they also make 25" wide bags, wow, but that was just a touch too much weight to justify). If it's really cold, cinch up the hood, turn your head to the side and wear a good hat -- I use a lightweight synthetic mad bomber type. Almost a perfect seal, and you can bring a fleece into the sack to lay over your arms up by your head to complete it.