How to keep the contents of a refrigerator from freezing in an outdoor refrigerator in the winter

place your scrumptious milk in a box that is luminated with plenty of insulation

We have an outdoor milk sales room on our farm. There is a half size refrigerator in there and we sell milk from it. In the winter the milk freezes. We are looking for way to keep the temperature inside the fridge about 35 degrees year round. We thought of building an insulated box to put the fridge in and putting a lightbulb in it to keep the air a little warmer around the fridge. In addition to this we were thinking of putting a garage refrigerator heating kit in it. Wondered if anyone knows of a refrigerator that is made to work when the temperatures drop. without boxing it in. Or if there are any other ideas out there.