This design is a variation of my spring loaded clip probe adapted for banana (4mm) connectors.
It uses a piece of wire (0. to 1mm diameter) a M3 8mm screw and a spring (up to 8mm diameter). A pen spring works well.
Follow the illustration in the pictures to bend and install the wire. The twisting with a drill is important as it not only straighten, but also strengthen the wire by work hardening.
I have tested this design with a variety of banana connectors and had no issue but given the diversity of existing models you should check the quality of the electrical connection for yourself.