hfss siw

Hi, I am learning Substrate integrated rectangular waveguide (SIRW) structure these days, and have a question to draw such structure in HFSS.

So I should only embed some same-height cylinders (vacuum) into the rectangular waveguide, that is it

OR

Besides above, I should also change the vacuum to some metal material such as silver, pec, etc. So the whole structure is like a wood with some pins inside.

Please clarify this. Thanks a lot