I’ve been a camper for 50+ years, and I LOVE this kind of low-tech ingenuity. Incidentally, #40 is even better in tuna or catfood cans; they’re somewhat deeper, so they burn longer. I can’t tell if they used wax here, but I do, pouring it in till only the thinnest edge of the cardboard sticks up, and leaving one slightly taller bit of cardboard sticking up as a wick to make lighting easier. This makes ‘em waterproof as well as long-burning. When set up on half a brick or a mound of earth, these burners will easily power a hobo stove made from an institutional-size tin can with about a 2”x2” hole cut in the lower (open) rim with tin snips or “miracle” scissors. and four smaller ones punched NESW (use a pointy “church key” can-opener) just below (not in!)the upper, closed surface. You can make one with both ends of the can removed, and many versions call for that—but this way you can fry eggs and such on the stovetop itself and don’t have to carry a pan!