Are you any good at sewing, or do you have a friend/relative who sews? Given the particulars of your situation, and that bringing her into the house is not an option, let's work with what you can do. I would get a couple of old blankets or matress pads from a thrift store, and then go to Wal-Mart and get one of the little emergency blankets that they sell in the sporting goods/camping department. They are about $2.00 each. These are reflective foil and they will reflect the dog's body heat back onto her. I would sandwich them between the two blankets or matress pads to keep her from trying to ingest them (not good on the digestive system). Even if they are covered up by layers of other stuff, they will work to keep it warm and cozy in the doghouse. Another suggestion would be to cover the inside walls with the stuff, but again, cover the foil with something else that she is less likely to try to chew on.
I would also get the heavy plastic you discussed to cover the door with as drafts and breezes can be enough to rob a dog's body of lifesaving heat. Good luck, and I hope these ideas help.