I bought a large sheet of drafting film (11 inches X 17 inches) from an art supply store for $1.40. This 'paper' is white plastic, allows much light to come through, and has no pattern that could be picked up on the baseboard. I cut a small piece and put it into my filter tray.
It even allows one to get away with using a regular 100 watt bulb (soft light kind) and, with a razor blade you 'file' off the bottom black writing (wattage, etc). Baseboard lighting is just as consistent as with a dedicated opal bulb. - David Lyga