EgineMode Dispersion Diagram with Open or PML Boundary

Hi to all,
I want to get a dispersion diagram of a periodic structure where one side should be radiation boundary.How can I get that with CST or HFSS
by using the eigenmode solver that don't support,both, radiation(PML) or open add space boundary?
I should use an air box on the top. How sufficiently large and what should I put on the top PMC or PEC?
Thanks