How large are the holes in your screen and are you certain there are no other holes in your bottom board? Carpenter ants can ruin a board in a few days.
It's window screen. The type you use to keep bugs out of your house. Most screens today are nylon or plastic but I just happen to have this role in my barn so I used it. The holes are a lot smaller that the #8 in a standard SBB. It also forces the SHB to come in the back where they fall into the oil.