Some people stick a battery-operated (small) fan into the fridge. I'm not sure how well that will work; the low temperature isn't great for batteries. It might be OK if the batteries are replaceable; you can charge one set while another one operates the fan, but I'd be prepared for daily battery changes. As an alternative, get a small computer fan and put that inside the fridge; these things are very cheap and draw little current, so you can run really thin wires through the door seal without creating too much of a temperature leak. (Feeding the wires in from the top seal is probably best.) Any run-of-the-mill power brick at the right voltage can be used to power the fan.