HFSS Fill Area with Material

Hi,

I am designing branch-line waveguide for millimeter-wave applications. I am doing this by first defining the copper structure and I want to fill the inside of the waveguide and branches with air. Currently I am doing this box by box and uniting them - very time consuming and sometimes the boxes intersect and it is really hard to find where the error is.

Is there anyway you can just fill an object with a certain material?

Thanks,
Torie