Anyone can give me an advise that is my result in CST

Hi All ,
Could you please help see the result of my simulation for normal WG fill-in with the vacuum,the BC is Ex=Ey=Ez=0 .The dimention a=37.5 mm b=75mm .I have quesion on how to create the geometry normally we must draw from inside material to outside material right? If i would like to construct the microwave oven cavity connect with the waveguide and have the material inside.If somebody have the same example like my requirement would you please help share to me.
Thanks and Regards,
Vincent113

Hi All Friend,
Anyone can help me?