I am sure you are done with this by now, but for future reference if anyone searches this topic.

You actually need 2 pre-made ones from walmart. One from pos to neg on the batteries and one for other pos to 60 amp circuit breaker. Then run your existing trolling motor wires from the breaker.

If going from 12 to 24 volts your wiring (if correct the first time it was installed) will always be heavy enough, as 24 volts will reduce the current so 10 gauge is good in most cases for 24 volt systems.