I don't have ice packs, but I have Bernat Blanket Yarn. I use it for this ice pack because I know how much water it holds. I've made many reusable water balloons with it in the past.

I made a square that would fit in a gallon baggie. Because of the thermal stitch, it's already thick. One layer is enough.

I did add a short video showing how to do the thermal sc stitch.

It can be done in your choice of stitches, however, what a great way to learn the sc thermal stitch. You want the thickness of a sponge, so another stitch would require you to make 2 and stitch them together. Or not, put them together in the baggie.