Please put an ice pack in the lunch box.
クーラーボックスに保冷剤を入れておいた。
I put some cold packs in the cooler.
冷却剤 is a more general term for any coolant or refrigerant, often used in industrial or chemical contexts, while 保冷剤 specifically refers to the consumer gel packs for keeping things cold.
Compound of 保冷 (keep cold) + 剤 (agent, preparation). A straightforward modern coinage for a common household item.