I frequently take my dog on trips with temps. below freezing. My method: 2 thin dog coats (pet store variety) +small "sleeping dog bag" + 1/2" thick EVA pad inside the bag. Bag is made from an old down vest with the bottom (waist) and arm holes sewed shut. On cold nights this combination keeps her from shivering and she is quite content.

You should be able to find a vest or (jacket) at a thrift store and can easily modify/sew this by hand--as long as your dog isn't too large!