I think your looking at the thermal requirements from the wrong end. The rate of heat loss through the insulated container is a function of the differential temperature between the inside and the outside of the container along with the thermal transmission coefficient of the insulation material. You do not need a heater that will produce 40 to 50 degrees of heat above ambient. Note that that phrase is totally subjective. You specify heat requirement in watts and once you've done the maths on the thermal losses from the container you'll end up with a figure for the number of watts you'll need for a differential of 40 to 50 centigrade degrees. Once you have the wattage, all you need is a simple wirewound resistor that will dissipate that number of watts when supplied with 3.7 volts. Of course that covers the worst-case temperature scenario so you will also need some form of thermal (proportional) control to to reduce the heating and so maintain a steady temperature within the enclosure if the external temperature is higher than the lowest expected external temperature.