The 3rd brake light cover is just to make it stand out a bit more and is legal as it shows more than 50% of the lights. I got this from eBay so have a hunt around.
Fitting is very easy - the only tool you need is the star screwdriver, size 25. Unscrew both of them on the back light. Insert the cover and screw back on. Do not overscrew. That's it. :)