Brillouin zone of a rhomboidal lattice cell - CST MWS

Hello to everyone!

I'm actually working on photonic crystals, in particular I'm trying to evaluate the photonic band gap, through dispersion diagram, for a number of different lattice shapes.

I've already successfully did a square lattice, but for now I'm unable to obtain valid results for a rhomboidal lattice, like that of the screenshot appended.

I know that, to calculate the dispersion diagram, the irreducible Brillouin zone must be scanned. This is fundamentally a triangle in the xy plane (I'm doing everything for 2D photonic crystals).

The problem is (I think) that CST automatically assign a cubic or parallelepipedal boundary box to the structures being simulated, and then one must set periodic boundary conditions in the xy plane. This is ok for a square lattice, cause the structure is perfectly contained inside the boundary box, but for a rhomboidal form some box volume remain unfilled and the periodic boundary condition lead to an inaccurate solution.

To this date i was unable to shape a perfect rhomboidal boundary box in CST and I think this is impossible because the simulator sets automatically the box outside the geometry.

Moreover, I also found impossible to assign master-slave conditions to opposite sides of the structure, like one can do in HFSS for example.

Any suggestions on how to simulate a structure like this? Did anyone successfully did it?

Thanks to everyone!