Dude, totally not worth it, when you can get a thermos that will keep your "fluids" warm for over 12 hours.
But if you're set on it, how about the heating element out of an electric blanket. it already has the temperature control on it. Then just wire up a secondary battery on an isolator so you don't run down your other battery. A deep cycle battery would prolly work best for this.
Or you could just spend $30 on a good thermos.