How many Watts do you need?
Will a children's riding toy yard car rechargeable battery work for you, dimensions are about 3"x 6"x 6" ?
I have done a lot of work with electric heaters over the past 45 years. You can buy heating elements of all sizes, shapes, lengths, watts, voltage, and special order heaters too. You can buy heaters with a built in thermocouple for temperature control. You can connect a 120v or 240v heater to a 12v battery and it will get warm. What physical heater size will work for your project?
You can connect a heater, on/off switch, fixed temperature thermostat and battery in series for a very simple circuit. A cheap low cost $3 thermostat will hold the temperature plus and minus 5 degrees F each side of the target temperature for a small light weight portable circuit.
This is a photo of a cartridge heater with threads, you can buy then with no threads, 3/16" diameter minimum diameter, length 4" to 20 feet long, any voltage any watts you like. You can by these on Ebay very reasonable prices.
Your not going to find may adjustable thermostat for heat that goes below 50 degrees. Look for refrigeration thermostats for lower temperatures.
**broken link removed**
Hot water heating elements come is all sizes and watts.
Check out this like for heaters.
https://www.google.com/search?q=hea...X&ved=0ahUKEwj-rfuH3u_JAhUCKWMKHUPsB3kQsAQIJw
This is a fixed temperature thermostat.