I've been poking around in some old posts and found this.
A friend gave me one of these a couple years ago and I have to tell you it is wonderful! I have not tried using it as a cold pack, but as a heating pad it is great. My one suggestion if you want to make these is not to use thin or synthetic materials. Mine is out of some stretchy fleecy type fabric and has had several repairs needed. I use it a lot in winter and the fabric doesn't hold up as well as I'd like. Other than that it, I love it.
Try it, you'll like it.
M