One degree on the low side is fine in my world. I would try to fill the empty spots in the fridge with containers of water. The more mass that you have in there the easier it will be to have consistent temps. I would not introduce a fan unless I had to. I use large igloo coolers for incubators. The first one I built I put a fan in, but never had to turn it on. The taller the incubator is the more likely you will need a fan, however, like I said, get some mass in there and it may even out.