#8 hardware cloth is wire mesh, 8 squares per inch, similar to window screen, but a little larger. We use it mostly for screened bottom boards, but also for robber screens and other uses. In a pinch, you could probably substitute window screen for a robber screen. Make the opening about 3/4" wide on the top bar of the robber screen, facing up; bees can find their way out but robbers usually can't figure out how to get in. 
