Fill irregular cavity with vacuum in CST Studio Suite

I am trying to simulate a 2.6-cell resonant cavity in CST, and I need to fill the cavity with vacuum. As far as I can tell I have completely sealed the cavity, but when I try to fill it with vacuum the surrounding space is also filled with vacuum.
How can I add vacuum only to the interior of the cavity?

don't know anything about CST Studio
can you fill space with vacuum and then add air back outside the cavity?