As mentioned above (reasons 8 and 9) there are those that make the challah round and flat for this Shabbos, in the image of matza.
Some make the challah in the shape of a key. Some attach a piece of dough in the shape of a key. Breslov Customs for Pesach (page 57) says this is the minhag of the family of Rav Elazar Kenig shlit”a and of Manistritch.
Sefer Hatoda’ah mentions making the image of a key with sesame seeds on top of the challah. These first three customs can, perhaps, be seen from the wording of the Ohev Yisrael in one place where he says to put the image of a key on the challah.
Others place an actual key in the challah. Perhaps this is done because of the wording in many places of indenting the challah with a key.
Either way it is done, the key or image of the key is usually on top of the challah. An interesting observation about this: The Gemara quoted by the Yismach Yisrael (reason 6), about the key, is at the top of daf 31b. At the bottom of the daf is the mishna mentioning the women’s mitzvah of challah. Here the key is on top and the challah on the bottom!
