I've remodeled the adapter from scratch because my old adapter (https://www.printables.com/model/98138-battery-adapter-einhell-power-x-change-general-pur), that was based on another design, has some issues. 

You will also need:

• 2 crimp spade connectors (e.g. https://www.aliexpress.com/item/100pcs-50pairs-6-3mm-16-14AWG-Female-Male-Electrical-Wiring-Connector-Insulated-Crimp-Terminal-Spade-Blue/32846771914.html )

• hot glue

• appropriate crimping tool

• wire

One photo (it's from an early iteration) shows how to put the spade connectors in place.

Improvements over the old version:

• easier to print in both orientations

• way better fit, the old one was too tight in my opinion

• the cables of both poles aren't strictly separated anymore and there is more room to be more flexible with cable management. Maybe even a small buck converter fits there

• indicator for plus and minus

Support recommend if printed as uploaded. The last photo shows how i printed it with support. I've used 4 mm yx separation from object.

If rotated by 180° no support is needed.

Cad files are attached. Feel free to remix.