I have had some experience with dry ice, as an instrumentation and controls engineer, I worked for the world leader in manufacture of dry ice equipment. When the Dry ice evaporates it expands as the co2 is heaver than air, the air rises, even if the lid was tight there would be enough pressure to force all of the air out of the deepfreeze. The DI will layer on the bottom and force air out of a deep freeze. A fridge will allow the co2 to flow out of any micro opening in the bottom of the door and would be ineffective. Picture this as similar to oil and water, the water settles out of the oil, as will the co2 in air.