Been seeing these distributed more as the weather gets colder; use a small candle or fill the innermost soda can with hand sanitizer, and they should burn for about 30 minutes. The flame burns hot and high; do NOT use them indoors/in a tent.

I met one of the people who made them and refined the design, and I got one to take back to replicate/make a tutorial for...

Supplies

Soda can
Standard tin can, around 15 ounces
Larger tin can, around 30 ounces
Can punch
Marker (optional)
Scissors
Pliers

Step 1: Punch Smaller Can

Use punch to make eight holes around the base of the smaller tin can. It's easier to get the spacing right by punching four evenly-spaced holes first.

Step 2: Mark & Cut Soda Can

Cut soda can to around 1.5 inches height; you can rotate the soda can with a marker held against it to get a clean line. The can is cut to this height as 1.5 inches is around the maximum depth of hand sanitizer that will burn reliably inside the can heater.

Step 3: Insert Cut Soda Can

Push bottom of soda can inside smaller tin can.

Step 4: Punch Larger Can

Punch eight holes along the top of the largest can. The edges of the holes should naturally curl up.

Step 5: Bend Smaller Can

Use pliers to bend edges of smaller tin can in. This keeps the soda can bottom from falling out if the unit is flipped over. Go slow to avoid tearing.

Step 6: Insert Can Unit

Place smaller can/cut soda can unit into the larger tin can.

Step 7: Secure Inner Can Unit

Use pliers to unfurl curled edges of holes in the larger tin can over the rim of the smaller unit. (You may have to redo step 4 with a can punch that makes longer holes if the inner can unit is not held in place.)

Step 8: Add Fuel & Ignite

Add a candle or up to 1.5 inches of hand sanitizer to the innermost soda can and ignite when safe. Once ignited, the heater seems to work by drawing air in from the holes in the top of the larger can, between the two cans, and up through the holes on the bottom of the smaller can. The flame is relatively shielded from wind and should be more difficult to accidentally put out.