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The best filler I have used for meat loaf is good old cream of rice cereal.
  Makes slicing very easy.  When I get a creative mood, I'll spread meatloaf
mixture on alum. foil; layer with gf deli baked ham, gf mozzarella cheese and
pull both ends of foil together (similar to a jelly roll).  Add your favorite
topping.   Bake in foil - when you slice the meat loaf  the melting cheese
and ham make a nice surprise.  Bon appetite!
Mary