Initially I used a piece of a pcb from old soundcard which happened to have the same contact pitch, filed it down to fit into the socket and attached my wires.
In the meantime I ordered a pre-configured cable with the necessary plug after a recommendation by @weigon https://www.ebay.de/itm/183626035646 which is a much better fit.
Too bad it seems my PWM control is not working at all, I'm beginning to think it is an electrical problem with the control circuit...
EDIT: looks like my Q41 Mosfet next to the connector is fried: On Q1 I do see appropriate changes of voltage according to the pwm settings, but Q41 is shortened...