Have you consiered using a transistor connected as a variable resistor?
Fix the transistor/s so the metal is the heatsink. Use a temperature sensor in the transistors bias circuit
(This is a good way to use up those power transistors that you buy at £1 for 20 and turn out to be out of spec - low gain / high leakage)
As has already been said previously, it's asking a lot of a small battery. It would need a large and heavy battery to produce any useful or worthwhile heat. It may be better to look at other energy sources.
Could it be possible to modify a portable gas soldering iron or butane powered tongs/hair curlers? The catalytic element produces heat without a flame and the item can be refilled with lighter fuel gas.