I originally stumbled across this great Instructable detailing how to make a straw slow-cooker, and was so thrilled by it that I soon made one myself. After the first attempt I discovered that it made a great incubator for home-made yogurt (I currently incubate my yogurt for around 24 hours for maximum probiotic action). As a result I modified the design slightly to easily accommodate a 2 litre jar. I find that 2 litres will probably last around a week for two people depending on what you are using the yogurt for. I like to drain it and make it into cream cheese and then make a tasty probiotic mustard with it for one thing.
The Hay Slow Cooker (hooikist) was invented in the Netherlands around 1900 and especially popular during the Second World War, when heating fuel was scarce. Nowadays it is forgotten, which is a pity as it can save up to 80% on cooking fuel costs. The Hay Slow Cooker is ideal in temperate climates for long cooking food in liquid such as beans, pulses, rice, grains, stews with meat like goulash, chicken pots like coq-au-vin, potatoes, porridge, soup, etc.. In warm climates a solar cooker is probably more practical. –kadewei