I'm interested in the prevailing custom (or customs) amongst German Jewry, and particularly among the Jews of Bohemia if there was a different practice there, for arranging the seder plate. What was the custom they followed?
According to Rabbi Shlomo Binyamin Hamburger, author of Shorshei Minhag Ashkenaz and founder of Machon Moreshet Ashkenaz, the Yekke minhag is to arrange the seder plate according to what the Rema states in the Shulchan aruch.
The Rema's arrangement is based on the fact that you should never reach over a mitzvah object to perform another mitzvah, so the things you need first are closest to you. Thus the salt water and parsley are closest you you, the maror and charoset in the middle, and the zeroa and beitzah are the farthest. Forgive the terrible ascii art, but:
beitzah---zeroa / \ / \ charoset maror \ / \ / salt water--karpas (you sit here)
Unfortunately, you'll have a hard time finding a Rema seder plate for sale (and if you do, please let me know because I've been looking for years). What I do is make my own seder plate by putting some bowls on a nice looking large platter.