I built this model to use Parkside battery with a Kärcher string trimmer (weedwacker).
It slides into the Kärcher tool, then the Parkside battery slides into it.
The battery contacts are made from Faston terminals:
The Faston terminals are crimped on two short wires.
Please note the polarity of Kärcher battery is reversed with respect to the Parkside battery. The wires in the adapter must be crossed.
Attached photos are of a prototype. Please pardon the ugly hotglue.
The author marked this model as their own original creation.