On Thursday, April 2, 2015 at 4:48:43 PM UTC-7, John Doe wrote:
Either corrosion resistant metal or a plastic. Something about the size of
a barbecue grill grating. But the grates could be narrow flatbar vertically
oriented. Looks like most barbecue grill grates are spaced almost 1/2",
that's too much spacing. Needs to be about 1/4" spacing.

How about expanded-metal lath (used for plastering)? You can 'tune'
the openings by stretching it a bit.

Otherwise, wire mesh 'hardware cloth' is a good solution (just a tad
awkward to locate). There's bronze-wire versions of this in the
bottom of most commercial sieves.