Thermal Simulation Software
Just use SPICE with resistor grids, plus capacitors at those nodes where significant heat storage will occur.

Use Rthermal of standard thickness (35 micron, 1.4 mils) copper foil as 70 degree Centigrade per watt. So use 70 ohms.

Inject heat, wherever you wish, as 1 amp (from a SPICE DC current source) per watt.

You have to define the size of the squares in the grid; you must then pick/compute the size of capacitor to store heat (if you include any heat storeage modeling).

How to model FR-4? That is about 100X worse than copper.

How to model air? That is about 100X worse than FR-4, depending on stirred/blown, or laminar.