Bulk sacks of rice, beans, grits, wheat berries, etc. are cheap, storable foods that have a lot of attractive qualities. The long cooking time, with resulting high fuel use, is not one of them.
I've seen recommendations about Thermos cooking in a couple of the preparedness documents I cited in the pandemic flu thread. They suggest that you purchase large, wide mouth Thermoses. With Thermos cooking, the idea is to get the food to a boil and then put it in the Thermos to continue the cooking process using the residual heat.
This is a poor technique. By transferring the food/water you lose the considerable heat stored in the pot and the Thermos is room temperature to start (unless you waste fuel by prepping it with a hot water flush) thereby initially lowering the temp of anything added to it.
A better technique is the straw box, which many of us learned about in Boy Scouts. The idea is to get the food to a rolling boil in a metal pot which is then taken off the heat and put into a box filled with insulating material. More material is put on top of the pot, the box is closed up, and the food is allowed to cook for several hours on it's own. The advantages are that the heat from the pot is retained and the food is not cooled by the transfer into the Thermos.
The ideal is to find a large styrofoam box such as the kind frozen premium steaks are shipped in. (Push comes to shove, a cardboard box will work OK.) This can be filled with any insulating material you have on hand -- even crumpled newspaper will do nicely. For best results, the insulation should touch the pot on all six sides.
Principles to be kept in mind are these:
* Insulation should cover all six sides of the box.
* The box should be airtight.
* The inner surfaces of the box should be of a heat-reflective material.
There are some adjustments involved in cooking with strawbox cookers:
* Less water should be used since it is not boiled away.
* Less spice in needed since the aroma is not boiled away.
* Cooking must be started earlier to give the food enough time to cook at a lower temperature than over a fire.
* The food should boil for several minutes before being placed in the box. This ensures that all the food is at boiling temperature, not just the water.
* Strawbox cookers work best for large quantities, as small amounts of food have less thermal mass and cool. Preheated stones could always be put in together with the pot to prove the additional thermal mass needed to keep the temperature up over a long period of time.