Frozen carbon dioxide (dry ice) clocks in at better than -100oF. So cold you need gloves to handle it. And it offers twice the cooling power and half the weight of frozen H2O. Lots to like about that except dry ice emits carbon dioxide as it melts. We’re not meant to breathe that. So if you transport via trailer or rack or otherwise have great ventilation, dry ice is a great choice. And dry ice should not contact the cooler surface, either. If you choose to use it, wrap it in cloth, newsprint or other material.