Re: "Tiny holes 300nm across are drilled into the metal"
"Can a virus get bigger than that ?"
Yes, but it's fairly unusual to be bigger than that in more than one dimension. The largest ones are aroun 400nm, and there are only a few know that are over 300nm. They're unusual enough that the first really big one to be found wasn't even recognised as a virus at first. None of the big ones appear to be relevant to humans; they're pretty much all water-borne and infect amoeba.
On the other hand, there are viruses that are long and thin, that can get to over 1um long. They'd fit through the holes lengthwise, but I don't know how this method would cope with long strings flapping about the place instead of neat little blobs that block holes up. That includes fun things like ebola and related viruses, so potentially quite relevant to humans.