Sorry, it was wrongly assigned:
Hello Schorsch, I have done a little research: There are so-called small car adapters such as the VOLTCRAFT SMP-40, but they only have an output voltage of 5 volts and not 5.2 volts. There is no guarantee that this will work, so you have to try it out. The original power supply of the Mi Box says output 5.2 V and 2.1 A. These are the basic values at which it works. These are the basic values at which it is sure to work.
The round connector has minus on the outside and plus on the inside.
Otherwise, you can only build one (or have one built), which is not difficult with ready-made converter modules.
The power supply unit supplied has a round plug and supplies 5.2 volts according to the imprint. It should be possible to find a universal power supply unit.