Does the out side temperature affect the smoker and smoking times? If so is there any specialist jacket/cover you could put round? Maybe a hot water tank jacket?
Yes it does, apparently there are some custom jackets and blankets for certain smokers, I'd suggest a quick google............they can't touch you for it Others have, as you suggest used hot water tank lagging.
When it's really cold (like it was sat night) I just start of with a few more lit coals. That's normally good enough for me - haven't felt the need to go down the jacket route personally.