NightHawk has a video on how to make cooling packs that keep their temperature around 18ºC (64ºF) for something like 2 hours. He gives the ingredients around the 6 minute mark. It's a mixture of Sodium Sulfate with common table salt:

• 5 cups water (1.2 liters)
• 1 cup sodium sulfate (240ml)
• 1/4 cup salt (60ml)
• 4 tablespoons of xanthan gum (20ml) - this is a thickening agent so that the thing becomes gelatin-like, which makes it easier to work with
• Mix in heated water for some minutes. The mixture should not fully dissolve, there should be some of it visible. If the sulfate fully dissolves, add more. If there seems to be too much of the mixture, add bits of water.

https://www.youtube.com/watch?v=Nqxjfp4Gi0k