Chris writes:

IF you want to go high tech and do have access to lab freezers (where
else would you get dry ice..., you can also 'supercool' your normal
ice before the trip. Ice at -70 C will last a lot longer than at -3 C,
with no extra weight.

For water, the heat of fusion is about 80 times specific heat, so
supercooling ice is hardly effective. You're much better off just using a
little more ice.

Dry ice is no good for refrigeration because it is too cold and difficult
to regulate to a higher temperature. By the time you build a contraption
to regulate the chill, you've lost any weight advantage over ice, assuming
you don't need sub-freezing temperatures.

I remember as a child seeing another child severely injured at my dad's
company picnic. Someone brought popsicles in dry ice, the kid got into
them, and took a lick.

Liquid nitrogen is a more practical wasting refrigerant since the liquid is
easy to throttle. That's what some reefer trucks use.