Step 1: Materials and Tools
You are going to need glue some sort of liquid nails use clear glue I didn't....
Sand paper, Paint(rattle can), Red Glass Paint.
Tools: Dremel with diamond Cut tip. http://www.harborfreight.com/5-piece-diamond-mini-cutting-discs-31501.html
Step 2: Breaking and Painting
Cut out with dremel tool and diamond bit approximately how much light diffuser you'll need if you have a corner to bend around use the soldering iron on top to melt through it enough that it will bend around the corner but not break and not leave a hole.
Put the glue all around the outside of the plastic casing and stick on the light diffuser. clamp it down using whatever and let it dry.