Measure the opening in the light fixture using a tape measure. Include the lip that the panel sits on when it is installed, minus 1/4 inch. The extra 1/4 inch will give you enough room to maneuver the panel into place.
Lay the panel on a hard flat surface. Use a straightedge, tape measure and pencil to draw lines on the panel according to the measurements from the previous step.
Heat the panel on the lines using a hair dryer set on the highest setting. Heat the panel thoroughly for 3 to 5 minutes. This will soften the plastic and make it less brittle when you cut it.
Cut the panel using a utility knife and by laying the straightedge on the line to ensure you cut it straight.
Install the panel into the light fixture. Notice any areas of the panel that don't fit properly and trim the panel using the utility knife to make it fit.