That looks really nice!
Where does your fridge give off its heat? If its one of the models that has the heat exchanger coils running on the sides of the metal box, then your just going to be heating the inside of your chamber. You can tell by turning it on and putting your hands on the sides of the fridge. If they are warm, your going to have to figure something else out.
You can still add a low wattage heat source and use it to maintain temps. It might fight with the fridge a little (ie run at the same time) so it could be wasting some energy.bring on the hot weather so i can see my baby in action! It went from 70 to 29 in one day! So now I almost need a heating element as well. But I cant add that, because my controller is cold only