We ship apple trees to some remote villages in Africa, and I try to include some useful agricultural information with the shipments. One of them is a cardboard solar cooker, as firewood is sometimes scarce and cooking with wood inside an unvented house is harmful to the eyes and lungs.
There were lots of newfangled designs available online, many of which required materials or skills that are not readily available in remote places. But one of the best and simplest designs I found is pictured above, the solar funnel cooker. The design is available at http://solarcooking.org/plans/funnel.htm It’s just a piece of cardboard twice as long as it is wide (bigger is better) with aluminum foil glued to it and bent into a funnel shape, with a hole cut in the bottom for the pot to sit. You put a black pot or big mason jar painted black into an oven bag and set it on a rock or cinder block to elevate it into the funnel.
I can tell you it works, as the first thing I did was to burn my fingers on the pot lid (D’oh!). It cooked a batch of brown rice in about an hour and a half. A mistake I made was to put the rice in the cold water before setting it out, so the rice soaked up too much water and was a little mushy; let the water heat up first before adding the rice to avoid this. Otherwise, it tasted just like rice cooked on the stove (the main question asked me by folks considering it). My wife ate it for breakfast the next day and said it was good.
I can tell you it sure beats walking 2km to try to find wood or paying for charcoal. It works especially well in arid areas that don’t have a lot of trees.