i used to have a piece of black posterboard that i spray painted black, behind a piece of plexiglass. but i wasnt really happy with it, so the other day, i masked off where the monitor is with blue painters tape, and spray painted the back side of the plexiglass with flat black spray paint. from the front, it has a very nice glossy finish to it, and it was cheap. ($5 for plexi, $1 for paint, $2 for tape)
i did the same thing for my control panel, i didnt want to mess with an overlay, so i painted the back of the plexiglass with flat black spraypaint, it gave it a very nice glossy black finish.