assigning Lumped port boundary condition(perfect H)

I have a parallel plate capacitor. There is a plastic box in between the plates, touching the plates. I uesd lupmed port to excite this capacitor and used one face of this box to assign the port. The boundary condition for lumped port is to have to perfect E for the conductor on both sides of the port and perfect H for all other edges.Should I assign perfect H to all 3 faces of the pastic box which are perpendicular to the capaciotr plates or just to the two faces which are in touch with face and the port? This boundary condition will change my result for H magnitude in the box. Please comment if you have any idea. Thanks.