Er, probably 99.999999999999999999999% of the screened bottom boards use galvanized hardware cloth. The other option is some specialty stainless steel or aluminum grids. O.J. Blunt down below Andalusia, Alabama was at one time selling sheets of stainless steel with round holes in it which were just about the perfect size for SHB. A couple of years ago he was selling the hive sized sheets for $15 each, which was inline with the price of the full sheets from the original vendor. He might still be selling them. Oj's an interesting fellow, I went to a bee meeting at his house a few years ago...he definitely loves bees!!!
Ed