I've done it once with mesh painted red, once with a Camry emblem stuck in it.....now its a actuall clear lense, and here it is.
Cost $9.00

Tools needed.
-Dremel.
-Cut off wheels *if your a novice dremel user, buy these in bulk! If youve ever used them...you know why.
-dry erase marker, or something that can be erased
-Phillips screw driver.
Materials
Clear panel for drop ceiling. You know the clear panels they use in office buildings, you can pick them up at any hardware store.
One L.E.D. bulb, RED. You can use other colors, at your own risk.
DIY instructions.
Grab both sides of the 3rd brake light cover and wiggle and pull *it will come out*
Unscrew the housing *2 screws*
Take light bulb out.
Remove the 3rd brake light.
Unscrew the whiteish part from the blackish part and remove the red lense. *2 screws*
Take the red lense and trace a outline on the clear panel.
Cut the clear panel out, and trim accordingly.
Put everything back together.
**Note** On some wedge style L.e.d. bulbs I found that they do not work from the package, you have to reverse the metal connectors...just bend them to the other side and try again. Now when you use a LED for the third brake light the brake light indicator will come on in the dash, I have yet to find a solution, so I live with it, and all the rest of my cool warning lights. The reason it does this is because the led uses so little power it trips the sensor to seem like the light is burnt. No harm will be done.