This is a good idea. I'd like to find one. There is an older thread on this topic.
I had the perfect 3rd brake light on my old Birkin. I got it at one of the local auto parts stores, but I don't remember which store, or which brand. It was a common thing at the time, but these things have since almost disappeared. Here's the one I had, mounted upside down and with the "legs" removed:
http://usa7s.com/vb/picture.php?albumid=60&pictureid=438
http://usa7s.com/vb/album.php?albumid=60&pictureid=438