I believe you will need Simulation or another FE package to do this. There is no need for a CFD code unless you want to account for cooling due to air flow in your box. You may be able to do this with a free FE package. Check out http://www.eng-tips.com/threadminder.cfm?pid=727 for threads on FE packages. I hope this helps.
In Flow, you can specify the location on the earth ( nearest city) and it will determine the solar radiation on your model based on the time of day (or by direction and intensity). I know it's supported solar radiation for some time, and in 2009 it supported objects that are transparent to radiation, so you can let radiation go through glass to heat up objects inside.
see attached image for interface
solar_radiation.jpg 92.4 KB