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Be sure to put a piece of cloth between the pack and your skin. I made one exactly like this and put it on my knee for only seven minutes. It felt colder than a commercial gel pack (better heat conductor?) and when I took it off, I had superficial frostbite: the skin was as stiff as cardboard. That was 5 days ago and it's still painful: please be careful!
This looks great, but don't the knots in the bottom of the laces dig into your foot?