Google "Keezer". I'm not sure how that approach would work with a side-by-side, but I have a chest freezer that is set at a constant 35 degrees using an external temperature controller. It basically watches the temp of the freezer and keeps it at a constant rate using a PID controller that opens and closes a relay that applies power to the freezer. I'm not sure how that action would affect the fridge side of the unit, however. They're not expensive, so maybe it's worth a try.