you can get a light diffuser(lighting dept) from any hardware store or also try an acrylic sheet/plexiglass and drill holes in them. sizes of holes is up to you or your drill bits.
I got one from Home Depot as a lid for the 50G growout tank, just need to top it off every three days (about 2G or so) Like Milkfish said, at any hardware store.
I read on DYI section of the forums that you can use. cross stich screens. You know those little plastic grids that you cross stch threw.
You can buy extra suction cups and tie them to the screens if needed.
Those also work good if you cut them in circles and put a larger hole in the center. They keeps the cats out of your plants dirt.