Could just do what I did in Kyrgyzstan recently - unthinkingly pack a one-season synthetic sleeping bag that's recommended for temperatures of 15 and above and get used to freezing your ass off when it's -7.
I have a ~£10 Tesco sleeping bag that's rated to about 15 deg and absolutely love it. Sometimes if my sheets are in the wash I sleep in it for a few nights because it's so great.
Could just do what I did in Kyrgyzstan recently - unthinkingly pack a one-season synthetic sleeping bag that's recommended for temperatures of 15 and above and get used to freezing your ass off when it's -7.
That may not be helpful.