I have had a bushbuddy ultra for a couple of years now, but if you fancy making a really cheap one from a couple of cans, this works just as well (though not as elegant I might add). The principle is to create a double skin allowing secondary combustion through the top vents. Ingredients: one big can of tomato soup (800g) and one smaller can of sweet corn to create the double chamber and inner firebox.
You will need to do some drilling to make the holes in the cans similar to the bottom photo before you fit the smaller can into the larger one. This should be a good fit, however, if there are any gaps between the two cans you can always do a bit of welding to form a really tight seal. You will also need to make a stand for the pot, you can make this by roughly cutting another can (about 1/4 of the can in length) so it can freely fit on the main cans. Cut a U shape into this can, so you can feed fuel into the main chamber once the fire is lit and the pot is sitting on top.Once fully operational it should take about 6-8 mins to boil 1 pint.