CST simulation of microwave cavities.

Hi everyone. I am doing a simulation research on microwave heating cavities.
I want to use the CST microwave studio to see the eletric field distribution within different shape of cavities,
but i am getting difficulties on building an rectangular cavity. I only can build a brick shape, but i need a hollow
rectangular cavity(with pec walls, and fill with air).
Can any one tell me the procedure of building a hollow rectangular cavity model? or any tutorial? Many Thanks~

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