Simulation Scenario

I have a sealed aluminum enclosure with electronics inside which are cooled via a heatsink with an outside ambient of 85 degrees C.  What I want to determine is what the temperature is inside the sealed enclosure will be.  What I'm having difficulty figuring out is what is the correct settings are for holding the ambient.  My approach was to set up an external simulation and set the outside ambient to 85, but could only find one place to set the external temperature at 85 and that was under radiation.  So I tried doing an internal simulation and setting the exterior walls to 85, but then I don't get the benefit of the enclosure for bleeding off some of the heat.

Does anyone have any suggestions?

Thanks

Keven