I used furring strips (1.5*1.5) w/ 1.5 rigid insulation, 1/2 plywood on the outside and some PVC sheeting on the inside. Basically made a box with an extension on one end that "slid" over the fridge on one end. Used some galvanized sheet metal to fold over the door opening so the magnetic strip form the door still worked. From ambient temps, it only cycles about 1/10 of an hour.
I used one of those "E-bay" temp controllers for heating & cooling. Only difference since this pic are the electrical connections for the heating & fan circulation inside the chamber. I used 2 old PC power cables and the receptacles off the power supply from an old PC. I added them to the left of the door.