Replacement fluorescent light covers are manufactured in standard sizes. Unfortunately, fluorescent light fixtures are manufactured in several different sizes. Plastic fluorescent light covers can yellow and become brittle as they age, prompting you to replace the old cover with a new one. Purchase a standard plastic cover that accommodates the size of your fluorescent light, and cut it to fit. Use caution when trimming the plastic, as the sharp edges can cut your hand.
Measure the original plastic fluorescent light cover. Mark the measurements from the old cover onto the new plastic cover, using a straightedge and a marker.
Lay the straightedge along the marked line. Score the plastic along the line with a plastic sheet cutter. Score the plastic sheet along the line six to seven times.
Break the plastic fluorescent light cover along the scored line by placing the cover on a table with the line just off the edge. With the scored line facing up, hold the plastic with one hand on the table. Apply downward pressure with your other hand along the scored line, splitting the cover along the line.