Just a suggestion here, if your having problems implementing a "Secondary" deflector too the top portion of the ship, you could do what the Galaxy Class has.  If I recall, the Galaxy Class has a small, imbedded Deflector in the bottom half of the Saucer Section.  This is mostly used when the Saucer Seperates from the Battle Hull, but it can be used as a Back up if the Primary Deflector goes off line.  Just an idea.