First I made the electric contacts for the headphones. For that I used copper tape.

First I applied it on the top; then I did several cuts, spaced 1 centimeter each, and bent them to form an electrode in the lateral. It doesn't touch my head because the headband has a piece of foam.

To avoid problems with the polarity, I added a diode bridge. Unfortunately, that meant that the output voltage would be about 1.4 volts less that the input, so I also added a voltage regulator. This allowed me to use a 12 volts power supply while feeding 5 volts to the headphones charger.

The next step was to design a support where to hang the headphones with two electrodes that would make contact with the headband. I used FreeCAD for that.

The file can be downloaded from the gitlab repository.

After printing it, I covered the tabs with copper tape.

Then passed the power supply cables through the holes in the base and soldered them to the tabs.

Finally, I used some hot glue to ensure that everything was kept in place.

And this is the final result: now my headphones are always charged and ready to be used.