Maybe buy some Glycol (very Cheap) and make you own eutectic tank. By mixing it at 3:1 you get a solution that freezes at about -10 Dec C. If the container holds a total of 4 litres then the stored latent heat is around 80 times what is required to change 4 litres of water just one degrees of sensible heat (sensible heat is what you read with a thermometer). Yes, 80 times.
Now pure water when frozen also stores latent heat but when it melts at Zero Deg C your frozen tucker has also (or is still in the process) melted. The benefit of using a Glycol mixture is that your tucker remains frozen because the glycol mixture is doing its latent heat transition at -10 Deg C.
You could also put one of these containers in a normal freezer to give it greater hold over capacity if turned off.
Read the article below to hear how stored latent heat can be used to its best advantage in a proper system that cycles off when the eutectic tank is frozen and cycles back on when the eutectic tank has thawed.
www.ozefridge.com/?page_id=22