You could connect two little 9V alkaline batteries in series for 18V.
Then connect ten 470 ohm resistors in parallel as a small heater.
For about 10 minutes the batteries and resistors will be too hot, after 15 minutes they will be warm and in only a half-hour the expensive batteries would be dead.
If you want a half hour of warmth then use ten 1k resistors in parallel as a small heater for two little 9V alkaline batteries in series. For about 10 minutes the batteries and resistors will be hot, after a half-hour they will be slightly warm and you might not feel any warmth after.
The little batteries are too small to power a heater for a reasonable amount of time, and their voltage runs down too much so the heat runs down too.
You could make a complicated "constant power heater" that would keep its same temperature for only about 15 minutes, but the batteries would be very hot for their 1st 5 minutes.