Just in case anyone is interested, this is all I needed to cut off to get a standard usb cable to work. The sheathing is the only thing that stops it being pushed all the way into the watch and it now charges the watch with no issues.
Do you have a picture or measurement of how much of the sheath will need cutting off? I've ordered my Notifier and it should arrive by the weekend but I wanted to see if I could modify a cable before it arrived so I had a second one.