HFSS doubt

Hello,
When we make a box in HFSS, we can assign a material.
but if we remove a face of that box, the box no longer remain the box.
it is shown as a sheets, it is not recognised as a object & material you have assigned is no longer valid.
so how to apply material to such object.
check image attached.

one solution i was following was make boundries of that box as a finite conductivity.
Is it correct way to do it?
if no then what needs to be done

I can suggest two ways
1. Instead of removing the faces, create two box and substrate small one from large box (considering thin wall in design)
2. Define perfect magnetic boundary on two faces which are open