I'm pretty sure my cables are longer than 30 inches but they only have a 'ball' on one end. I'm sure something can be fashioned, or clamped on the remaining end. I'm assuming that your cable was one piece from the factory? So maybe they fed the cable through then put the ball or other end on after it was fed through the housing?

I'm thinking a standard brake cable with a straddle cable clamp on it.

A google image search gives this as an example. $11 is insane. We used to have tons of these lying around and I'm sure you can get one from the LBS for free or a couple bucks if you rummage through their junk bin.

http://www.jensonusa.com/!D8YfPO5HzvLGu4Yjp6UTUA!/Jagwire-Standard-Straddle-Cable-Carrier

Clamp that on the cable and use a dremel or something similar to cut that square hole for attachment. If a bike brake cable is too thin get some really thing cable from the hardware store and put a clamp on both ends. With a dremel, pliers and/or file you can rig some thing.