What ? Woodstove for my garage. It looks like one Futurama protagonist: Bender!
Why ? Because German winters are rude!!!
How ? I first unscrewed valve holding upside down bottle on vice and using a metal bar to turn bottle.
WARNING: I fully flooded the gas bottle with water in order to be sure all gas inside is gone.
Then I could safely cut the bottle with a cutting wheel. Despite my cheap MIG welder, I could get nice results on welding. I reused the cut piece to make the door and an old circular saw disc to create 2 chambers. Second chamber as a fresh air inlet to involve second combustion and enhance stove efficiency. I didn't quantify gain and it could be interesting to do it... A potential improvement would be to enhance convection outside to heat up the room more quickly.