Spawn a Matrix of Objects over a Heightmap

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I'm trying to spawn a matrix of objects to create a field. Ideally this could be done while the model is running so I thought using a plugin was the right strategy. I successfully modified this plugin http://gazebosim.org/tutorials?tut=pl... to place a model at the correct height, but I cannot figure out how to change the name so I can spawn several of the same model. I used the factory portion of the tutorial (option 3) as it will be most convenient to store models in a directory and then call them and place them at the correct location. The main code is copied below. I tried several iterations of using msg.set_clone_mutable_name and some other options with no luck.

         //  Insert model from file via message passing.
    {
      // Create a new transport node
      transport::NodePtr node(new transport::Node());

      // Initialize the node with the world name
      node->Init(_parent->Name());

      // Create a publisher on the ~/factory topic
      transport::PublisherPtr factoryPub = node->Advertise<msgs::Factory>("~/factory");

      // Create the messagecatkin
      msgs::Factory msg;

      msg.set_sdf_filename("model://Cylinder2/");

      physics::WorldPtr world = _parent;
      physics::ModelPtr model = world->ModelByName("heightmap");
      physics::CollisionPtr collision = model->GetLink("link")->GetCollision("collision");
      physics::HeightmapShapePtr heightmap =
          boost::dynamic_pointer_cast<physics::HeightmapShape>(collision->GetShape());

      /* Spawn a matrix of cylinders, doesn't work without changing name.*/
      for (int i = 0; i < 10; i++)
      {
        for (int j = 0; j < 10; j++)
        {
          //  getting the height :
        float z_value = heightmap->GetHeight(i, j);

        // Pose to initialize the model to
        msgs::Set(msg.mutable_pose(),
                  ignition::math::Pose3d(ignition::math::Vector3d(i, j, z_value+0.5), ignition::math::Quaterniond(0, 0, 0)));

        // Send the message
        factoryPub->Publish(msg);
        }
      }