Hi Daniel,

A simple additional boundary condition is the bottom of the box. Assume the box is sitting on an insulated surface.

For simplicity, let the cool product on the inside be a cube sitting on the bottom of the box in the center with air all around it.

In DesignModeler, create the cube solid and the box solid, using Add Frozen, so you have two separate solids. Then use Boolean to Subtract the cube from the box, using Keep Tool so you have two solids, the cube and the box with the cube shaped hole in the bottom.  Now select these two solids and Form New Part to share topology.

This is about the simplest geometry you can make to do this simulation.  The box solid represents the air inside the box. Five faces of the box can have a convective boundary condition with the warm outside air temperature. On those five faces, you can also define a wall thickness and conductivity for the box material. The cube solid represents the product, and you will need to define that as a solid with a specific heat capacity and give it an initial temperature. The air inside the box can have the same initial air temperature.  You will want Gravity included to allow the natural convection to occur inside the box.

You will also need to imprint a small square face into the side of the box solid to represent a pressure equalization hole. As the air inside the box warms up, it will expand and a small volume will need to exit the box. The face on the side of the box is defined as a Pressure Outlet boundary condition.

Here are a few relevant discussions.