Even though it seems strange to stalk with such a bulky shield, I would not add a (negative) modifier to stalking maneuvers. The problem I see is that, as a GM, I would soon have to rule the modification for all kinds of large objects a character might be carrying in his hands while stalking, be it a spear, a normal shield or whatever. Therefore we are only using the modifications from the official maneuver tables list for stalking. Since these IIRC do not contains a modifier for carrying large objects I would not add a new one.
Just my 2 cents