I made the whole thing in the past few days, and it was ready by yesterday's afternoon. Tested it ofc. It took a bit less than an hour to warm up 1 liter of water to almost boiling point. I used a tin can also spray painted for the test. It started at 15:20 and called it at 16:30. In the last quarter hour not much happened, before that bubbles started to form on the inner surface of the can, and the water gently moved. I also covered the can partially with a piece of broken glass. Fully covered, it might had really boiled. Since the test started after the ideal hours, I still call it a success.

According to the manual, the Sun-Funnel Cooker was designed for a 4 liter pot. So the 5 liter keg is a bit oversized. But we'll see.