the problem with using a can as a windscreen, at least with beverage type cans, is that the sidewalls of the cans are very thin, and most of the can's strength comes from the top and bottom rims, which are thicker.

You will definitely need ventilation holes on one side of the bottom, this is obviously the side you will face away from the wind direction.

You will also need to leave at least a 3/8" gap around the pot for the heat and exhaust to escape. The closer the gap, the more efficient the heat transfer, but too little gap will either suffocate the stove, or cause the fuel to condense on the pot. Both bad. The good thing about using something like cans is that they're cheap, so you can play around with it, and experiment with different ideas.

The other problem I've noticed if your windscreen is too close to your stove is that it will reflect too much heat back into the fuel, and speed up the burn rate, which sometimes reduces efficiency. I've done a good deal of tinkering with my alcohol stove of late, and was able to get my pot/stove/windscreen setup weight down to under 2 ounces. I can boil 2 cups on less than a half-ounce of fuel. In relatively calm air, my boil times were in the vicinity of 9 minutes, not fast, but for the overall system weight, and fuel efficiency, I'm fine with it. I also tested it in 20+ mph winds, and was just barely able to get a full boil, which is pretty acceptable performance in my mind.

The pot is a foster's can with the top 1/4 cut off, and a lip rolled into it for sturdiness. The windscreen is a DIY caldera-cone type setup, and the stove is a tealight cup. Total weight for the setup, plus my styrofoam drinking cup is 1.9 ounces. I'll try to snap some pics of the setup, and maybe make a thread about it. I was able to shave something like 6-7 ounces of pack weight over my old stove setup, at the expense of slower boil times, which I'm more than ok with.
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