I broke the o-ring between lens and bezel. The new one makes it nearly air-tight 
. I’m about cutting it up again.
Also, it is not perfectly in focus. The LED should be raised or the retainer lowered. Don’t think it has a great effect on brightness tho.
I applied a small amount of thermal paste.
After 60 s:
With thermal paste: 36,5 °C (initially 23 °C)
Without thermal paste: 33 °C (initially 22.3 °C)
50 °C are reached after 2’34’’ now (in temp limit adjustment mode).
Brightness [a.u.] vs. time [s]
Temperature regulation (50 °C max.) kicked in early but was off by +9 K. I couldn’t comfortably touch the light. Nearly 80% brightness for the first 100 seconds is quite good imo. The cell had more than 3.7 V left after 6 minutes.
I think this is all ok somehow. Toy is in its final state.