As an alternative to both plastic (nylon) screen, or normal metal screen, you could ask around at any place that does repair, or servicing of semi trucks for used air filters. They wrap them in a sheet of perforated/drilled stainless steel. And the holes are spaced widely enough that you could drill them out some more if they're too small. I'm using the same material as a guard for the exhaust pipe on my bike. While I think of it, the exhaust guards they use on semi's are just thicker chromed steel, or stainless steel with larger holes in it. That might have some potential use for gold panners too.
F.