If you go back a little from the link I posted, you can buy single sheets. You are going to lose a good amount of lighting by using the black eggcarate though. The white and silver eggcrate help to diffuse light but I think the black will absorb a significant portion of the light. Pond netting in a window screen frame will serve the same purpose and won't block much light. There are some threads on here explaining how to make them.