Zen Stoves, bottom of page for cone calculator.
You can buy aluminum flashing by the foot, a foot wide should do it if I remember right, at Ace hardware, 12" high costs 89 cents a foot. Important: they coat flashing material with a sort of plastic type coating, so it's a good idea to sand it a bit to get that stuff loosened up, and the first burn will vaporize that sanded material, so don't do it inside, until it's burned off. You can also burn it beforehand by passing it over a stove gas burner if you have gas stove, ventilated of course, then sand it down with emery cloth.
The main thing to watch for is avoiding stoves that burn too hot, they will roast the top edge and ruin it, so you need the most efficient type stove you can make or get.
If you add 1/2" to each end and fold it over so you can slide the two parts together, you get a nice easy to assemble unit. But make sure your stove is burning very slowly before you start, I have usually ended roasting my flashing to a crisp because the stoves were too hot for a cone.
Re the time it takes to make this stuff, that's a positive, not a negative, it's time I'm not working, and it's a much better thing to do than work as far as I'm concerned. Ie, that's what hobby's are for.
Ignore the stuff about the patents, if you are making a copy for yourself, and not selling it, it doesn't matter at all, I know zen stoves put that in there just because they felt they had to for legal reasons, but in the real world, it doesn't matter. As with free/open source software, the issue actually only kicks in on distribution of the work. Plus, as zen stoves noted, some of the current patents granted were created using the data on this very zen stoves page, only nobody thought to search out for prior art, so the generally sloppy and lazy US patent office granted the patents without really checking into it, just as they do all the time with software patents.