Materials:   

1.  One meter Cable with 5.5 mm x 2.1 male connector.  Conrad 1.07 EUR   A 90° male adapter would be better than the straight, if you can find one. 

2.  9V clip battery clip with removable cover  Conrad 0.40 EUR (there are others from other suppliers).    The Conrad part is a soft plastic and melts more easily when soldering

3.   Solder and soldering iron.

Remove cover from 9V clip

Remove existing wires from clip.

9V clips with crimped terminals can have a nickel coating, which makes soldering difficult.     Remove nickel coating with a small stone wheel, or round tipped file.   .

Plastic clips tend to melt easily.   Use a battery as a heat sink while soldering, else your terminal might melt through the plastic!

Tin the terminals with a little bit of solder.   Be careful of how much heat you use and how long it is on the clip.   They melt easily!

Thread the cable through the center slot.  The  stripped (+) cable goes on the crown shaped terminal on the clip.   The all black (-) goes on the round terminal of the clip.  (the battery of course is just the opposite)

Quickly solder the wires onto the terminals.   Have something to hold the wire in contact with the  terminal so you can do this quickly, with little heat.  

Reassemble the cap.   Test the connections for continuity.