Dave,
The foil-sided (both sides) water heater insulation blanket is perfect for this. I smoke all-nighters in below freezing temps and have found that the blanket is awesome! I do not wrap it against the smoker. If you are using Digital Temperature probes though, be careful, the contacts in the transmitting components will freeze. Place them in a insulated cooler! I learned that one the hard way!! That was also when I learned to NOT wrap the blanket completely around the whole cooker - got way to hot!! Good luck!