Extending Servo Wire Answered

I need to make a servo wire that extends approximately 4 feet and then retracts to a length of 18 inches or less. There is a risk of entanglement, so the less sag and less loose the wire, the better. One option would be to use coiled wire of the appropriate gauge, but a product such as this is expensive. What are some other options?

One idea that I had was to build a wire spool mechanism like that of a tape measure. From what I understand, a tape measure uses a constant force spring to somehow apply a retractive force to the actual measuring tape. Can anyone provide a more clear explanation of how a tape measure retracts? Does anyone have any ideas of how to implement this into a wire spool?

Other ideas on retracting a very long wire?

Thanks