CST - Pattern inversion

Hi,
I am trying to do the following:
1. take the mechanical solid design of a body (say a complicated waveguide power divider+polarizer+ transformer etc.)
2. generate the NEGATIVE to find the inside solid
3. import this new solid in CST for analysis with boundary conditions PEC
I don't know how to to the NEGATIVE in step 2.
Any suggestions?
I suppose people who do castings should need the negative to do electroforming on it, or just to produce a mold.
Anybody knows what software they are using?
I tried to do it in autocad. I just generated a big cube and cut out the positive out of it. But then I am left with what I need AND the rest of the cube, which Autocad does not allow to get rid of.
I tried to use the SEPARATE command (in Autocad) without success.
Thanks
Kato01

I'm find that CST4.0 can not export own structures in AutoCad extension in 3D, only in 2 d and with the error. This is new option ver 3.2 have can not import dxf file and probably that's the reason for this bug. I'll soon check this in ver 4.1 and send my results.

In my opinion, the Negative step (step 2) is really not necessary. If the solids you draw are all PEC, just import them into CST, set the background material to be free space, set the objects imported to be PEC. Then CST is "smart" enough to know that the fields needs to be solved are in your "Negative" region. Similar thing happens when using HFSS.
Because the computation domain in the CST is alway a Cube, my guess is step 2 wouldn't help you save memory either. Is MWStudio smart enough to recognize a lot of cells are just PEC and not allocating memory for it? In HFSS, by default the fields in internal region of a good conductor is not simulated.
I am not very familiar with CST, I could be wrong.