DIY Gel Ice Pack ~ (Need to try this soon) Liquid Dish Detergent + Ziploc Freezer Bag (double bag as extra insurance against leaks) ~ Squirt liquid dish detergent in a ziploc bag until the bag is to full, seal and freeze. re-freeze-able.
Home Made Gel Ice Packs - This is awesome! My back got sunburned so I got one of those huge ziploc bags they have for storing toys and made a huge version of this and laid on top of it. Water, Rubbing Alcohol, and food coloring
How To Make Your Own Shapeable Ice Pack For Pennies