To undertake manufacture we recommend that you first thoroughly understand the principles on which the cooker works (see Research Results).

Then:

1. Choose your cooking vessels from among those that are readily available in the market. The vessels must be stackable one over the other without too much of an air gap between them. They should not be a tight fit as this makes it difficult to remove individual vessels later, when they are hot. Too loose a fit means that the fuel savings associated with "multi-stage evaporation" will not be realized. The material of which the vessel is made must have high conductivity. We recommend stainless steel. We discourage the use of aluminium for cooking as this represents a health hazard.
2. The cooking vessels can be of two types:
   • constant diameter but varying heights (if so desired) where the bottom of one fits into the top of the next.
   
   vertical fitting
   • nesting diameters, where the vessels nest inside each other, but if placed in reverse order they stack, with the smallest vessel at the base and the largest at the top.
   
   stacked inside
   
   pots and lids
   
   stacked
   • Our more recent research suggests that it makes little difference whether the upper vessel sits directly on the rim of the lower vessel, or whether the two are separated by a flat stainless steel lid which is wet on top. Conductivity through the lids is excellent and the presence of the lids hardly affects thermal efficiency. When using lids between vessels they can be stacked in any order, with the largest vessels below and the smallest on top probably offering the most stable stacks. Alternatively, all vessels can be the same size, if stackability when empty is not a consideration.
3. The topmost vessel needs a slightly convex cover. Its purpose is to make sure that any water droplets that condense on top should fall back into the base of the cooking pot. They should not enter the food in the topmost container, and water should not also accumulate on the top cover.
4. Because they are stacked, you will probably find that an assembly of wide diameter, shallow vessels is more stable, and looks better than the same volume in narrower, deeper vessels. But in general, the taller the stack the more efficient the cooker, because you reduce the heat-losing surface area per unit of volume enclosed. On the other hand, the diameter of the base needs to be large in relation to the size of the flame, so that a minimum amount of heat is wasted up the sides. Finally, when choosing the vessel sizes, you need to keep an eye on their final acceptability to the purchasing public.
5. The next step is to design the steaming chamber. If you can get a readymade bin with a lid, this will do. At least the base needs to be of metal because it sits directly on the fire. It should be as compact as possible, that is, it should just contain the stack of cooking vessels. With too large a steaming chamber you are only increasing the fuel requirement to heat up the chamber, and increasing heat losses from the sides and top, not to mention increased cost. If it is a bin, you will have to have a cradle or other device for lowering the stack of cooking vessels into the bin, and pulling them out once cooking is over. The lid should be a close fit, but not too tight, as we do not want a build-up of pressure in the steaming chamber. Once again, care must be taken with the details of the way the lid fits on the bin to ensure that steam condensing on the lid finds its way back into the base. In the EcoCooker we have eliminated the cradle by making the break between bin and lid not near the top, but near the bottom of the steaming chamber. Where the break is makes no difference to the performance of the cooker. But you need to ensure two things:
   • that the lid fits well, but not tightly, so it can be easily removed. Too loose a fit is undesirable because too much steam should not escape from the chamber; and
   • that water condensing on the lid finds its way back into the base and does not leak out of the steaming chamber.
6. A fundamental requirement is that the water in the base MUST be in contact with the lowest vessel when heating begins, and MUST NOT be in contact when the heat is switched off an hour or so later. If it were to remain in contact the lowest cooking container would cool too fast, and the food in this container would remain incompletely cooked. You will need a stand of appropriate height to set the cooking vessels suitably above the bottom of the steaming chamber. To determine the height of the stand, a good thumb rule is that the water in the base should be about 7% of the total cooking container volume. Ideally, the volume should be such that when the cooker is opened there is still some water in the base, not counting the water that will collect there because of steam condensation during cooling. In other words, the base should always have some water in it, and should never dry up completely. A simple test would be to put tiny pellets of wax to float on the water in the base. When the cooker is opened, if the pellets are found melted, then the base did dry up completely.
7. If you choose not to use a stand for the cooking vessels, but have them rest instead on a step inside the base (as in the domestic models of the EcoCooker), you need to make sure that there are enough and large enough steam vents to allow steam to escape from below the bottom cooking vessel into the steaming chamber. In an early model of the EcoCooker these vents were too few and too small. The result was that the steam build-up expelled water out of the cooker altogether, the base dried out, and cooking remained incomplete.
8. The final step is to insulate the steaming chamber. Use any method that appeals to you. We found most insulating materials either too costly or too much of a hazard in the kitchen. We finally settled for air as the cheapest insulator. The ideal air gap is about 5mm, too small for convection currents to develop. The outer cover is metal, because it has to be rigid, unbreakable, easy to clean, easy to handle and cheap. If you have chosen a bin as your steaming chamber, you will need an inverted bin that fits over it to provide air-gap insulation. If your manufacturing process allows it you can combine the wall of the steaming chamber and the outer, insulating wall into a single double-walled vessel, with air or an appropriate insulator in between
9. You need to provide some way of handling the cooker if you want to take it off the fire and put it aside during the cooling period, when it is still unopened. If your steaming chamber is a bin, attaching handles to the bin at a level higher than the center of gravity of the assembly solves the problem. In the domestic models of the EcoCooker, where the break between base and lid is near the bottom, we found the least complicated solution was two pairs of handles, clamped by the grip of the person lifting the cooker.