Please also note that pinouts should be matched when soldering the both parties, so pay attention to the numbers on the plastics of female (socket) and male (plug) connectors. The pinout numbering is standardized on all types of connectors.
The above illustration shows the frontal (not soldering side, but connecting side) drawings of the plugs. Sockets are mirrored horizontally to match. Numbering eliminates the soldering work in a wrong way.
The green colored pins are reserved for 3.6V DC (exposure meter power supply).
Since the external battery packs will be separate, its advised that the matching connectors with bayonet or screw lock should be considered at first for added mechanical connection safety. But this needs a replacement for the original socket on the camera body, and also a replacement for the battery handgrip plug may be needed, when switching from external pack to handgrip, vice versa, is required.
However, this safety option has a drawback: If the camera user and/or battery carrying person forget that the external battery is connected to the camera via a cable, then bad things may happen, especially when the camera is on a tripod!
Spiral cables may be a good selection.