If we talk about a third brake light for everyday use, which is indeed concealed by a suitcase but does not irritate the nice shape other than using a cyclop brake light I found Jochens solution very good. It is LED and does not need too much additional current.
Here are some pics.
http://www.talkmorgan.com/ubbthreads.php/topics/430090/Replacement_"Cyclop&a#Post430090
