Round-Rod Combline Band-Pass Filter in HFSS

Here is the video. Sorry for the Malay language!

https://www.youtube.com/watch?v=3EuE9lq7Heg

So I was able to copy this design, and my simulation results
were pretty much the same as hers. But I have some questions, as
usual:

1) The main filter box was simulated as a "vacuum", and was never
converted to a perfect electrical conductor (PEC). The outer coax
cylinders were also vacuums, which is why it didn't matter that
her inner coax cylinders were not perfectly centered to the outer
coaxes. It looks like the cylinders in HFSS have disks on their
ends (they are like cans, not tubes!), so if I were to convert the main box, and
the outer coaxes, to PECs, I would have to subtract these disks from
both ends of each outer coax cylinder, and from the box itself, right?

2) I was under the impression that an excitation port had to have a
opposing ground-plane, but it's clear this is not the case, because
this model had the outer coaxes as vacuums.

3) Does anyone have a link to a white paper, or online calculator, or just a
website, that can show me how to generate a round-rod bandpass filter
like this one from scratch? Given the center frequency, the -3dB cutoff
frequencies, etc? I was not able to find a good one.

Thanks in advance.
Paul