Fill a plastic bottle about 3/4 full with water and leave it in the freezer for several hours until completely solid. After you've made your soup and are ready to pack it away, let it cool for a few minutes in the pot (so it's no longer boiling hot). Then gently stir it using the frozen water bottle.
It will take different amounts of time to cool your soup depending on how much you made. You can leave the water bottle standing in the pot while you do something else, but be sure to stir it every few minutes to agitate the hot and cold spots. Once the soup feels cool to the touch (that is, just below body temperature), it's fine to put in the fridge.
So far, we haven't had any problems at all with the normal plastic water bottles melting - this was definitely a concern of ours when we first came across this tip years ago, especially considering the concerns with heating plastic. But the frozen water inside seems to keep the surface of the plastic cool enough to prevent any melting, and in fact, the bottle actually stays fairly cool to the touch the entire time.
It's still a good idea to double check your bottle as you're using it. If the plastic is starting to feel soft and pliable, don't use that bottle!
You can also look into buying professional food-grade ice paddles. These typically start at around $15 for small paddles and increase in price as you go up in size.
• San Jamar 2-Liter Rapid Cooling Paddle, $16.99 from the Webstaurant Store
(Image: Emma Christensen for the Kitchn)