problem in finite FSS simulating use HFSS

In a infinite structure we know we can set BC as master & slave and floquet port to analysis, but when I want to analysis a 5*5 structure
How should I set the BC and what port can we use? Because I need to create a TEM wave, and when we use radiation BC and wave port can't create a TEM wave.
So hope someone can help with this question or sent me a finite FSS model is ok.

I am interested in finite arrays as well. Have you found the answer to your question?

if your array is finite (5*5), then you should use PML for all 6 walls and apply plane wave to hit it.