Pure rubbing alcohol should not freeze and can be used to clean the rifle. Another thing you can do is take thermos bottle of boiling water with you. By the time you come around for cleaning, it should be still warm if left in the car. Another thing you can do is get a small bottle of pre-lubed cleaning patches like Rusty Duck sells and so does Thompson Center. Just tuck them in your coat pocket. That body heat should keep them fine plus should you need them in the field for a field cleaning you have them.

All else fails, clean the major fowling out with dry patches and spit. Then when you get home your cleaning chore should be much less...