After the tank was ready, I couldn't await the first fire tests. To do the tests I needed a rack, which I built as a first draft out of old pallets. This was better then nothing, but not satisfying in a long term.
Then I started with a burning test. I used some coal brickets and later wood to test how the air flow works and if the chimney is functioning.
The results:
Wood or coal should be heatened up outside, for example in a charcoal chimney.
The airflow was good when the temperature was high enough as the chimney sucked enough air then.
Without heating it up externaly, you need to actively blow air into the box (like the lotus grill does).
Without any additional measure, the box can hold a constant temperature of 160°C.
=> So all in all, it sounded promising. Seems like it is realistic to get a temperature range between 50°C and 200°C in the box, with higher peaks when grilling above the blaze.