Amar, it looks like what you're describing is a rectangular piece attached to part 74811 in this diagram:
I couldn't see a separate listing for it, so it may come as a piece of that part, which is called "Cover-engine, lower". The part is listed
here for $109 ordered, but you may want to ask around about that part. Maybe pm FuszNissan or FBNissan about the part itself to determine if that is the exterior part, and if the rectangle is a part of it.