right now my primitive bed rool is a shell of canvas. a wool blanket bag and a bag of modern felt. it worked well at our last service project. we doing streambank restoration during the day. and it got bitter cold at night. not as warm as i wouldbe with a modern bag and pad. but not terrible.
I second what Whisper was saying. make yourself a good ground bed of dead leaves or grasses before you bed down. you loose a lot more heat to the ground than you do to the air. so insulate yourself from the ground and youll be much warmer.
here is a pic of my bedroll.
ive redone the straps recently and also built a rig to carry it on my shoulder. ill try to get newer pitures.