Reduce battery voltage.
Adding a resistor half the resistance of the elements will get you down to 12V, and increase battery life by 50%, but you end up wasting a third of the energy in the battery (unless you use that resistor to keep something else warm). There are cheap DC-DC buck converter modules available, that will regulate down to 12V, that then gets your battery life to more than double - power at the element falls by 56%, power consumed at the battery falls by that less the efficiency penalty of the converter.