@Joey Hess I'm not sure what your target temperature is but if it's not 0 C but is between -46 C and 17 C then you can use a cheap trick to improve the performance of your water jug buffer. By mixing glycerol and water in different ratios you can adjust the melting temperature to anywhere between -46 and 17 C. If you adjust the jugs to melt just above your target fridge/freezer temperature then the temperature will stop rising when they hit that temperature and stay stable until the water-glycerol mix has all melted.
There is a bit of a trade-off involved:
- Enthalpy of fusion Glycerol: 200.62 J/g
- Enthalpy of fusion Water: 333.55 J/g
- Specific heat capacity of Glycerol 2430 J/g/C
- Specific heat capacity of Water 4148 J/g/C
So you might want to take this into account when picking a target temperature.
You can find the melting point for different ratios of glycerol to water in a table in this paper from 1925 (Freezing Points of Glycerol and Its Aqueous Solutions by Leonard B. Lane).