I'm going to second Tom on this one. Buying a shorter cable will not help take up space in the boot, nor will it effectively wrap the shell around the ankle area if the extra volume inside is too great.
Your best bet is to take up space by putting a shim underneath the liner. You can cut a 3-5mm thick piece of flat plastic or cork material or at the very least cardboard. Use the sole (bottom) of the liner as a template and go slightly shorter (5mm), slightly narrower (2-3mm) than the liner bottom and you should help until it's time to get new boots.