Yes - the emisivity of screen within the wavelength concerned certainly has major impact. The is why we wrap our buildings in foil to reflect some of the infra-red radiation and apply low-e coatings to glass to assist with heat loss/gain. Cardboard = no chance (virtually) metal = high chance of reflecting......microwaves in this case.
Ok - I think I worked this out by drawing it. Please correct me if I wrong...
A series of photons approach the screen. Their wavelength is 10x the size of the screen openings and the screen is 30% open. 30% of The photons do pass through the openings (the others hit the screen solid and reflect back) but they pass through every opening and arrive at a point on the side with varying amplitudes which then deconstructively build giving a very limited probability that they will be found there.
When the wavelength is changed to smaller than the opening we get a higher probability amplitude directly in front of the opening and deconstructive ones at holes further away.