It's not easy to understand what you want say.
Is the Shielded structure a waveguide, or a mirror or what?
generally speaking the roughness play a role smaller that many people thinks.
For a Waveguide the roughness increase lhe loss (not so mutch as you change the metal from Ag to Al). For a mirror a gaussian distributed roughness generate an effect like a scatter (reflect to every direction in a half sphere). The amount of scattered power (the solid angle integral) is:
1-e^-(4*pi*r/(lambda^2))
where e=2.7182... , r= roughness rms, lambda=3E8/Freq(Hz).
next time, please be more explicit.
have a good day