private void CloneLayoutUpdated(object sender, EventArgs e)
{
    Rectangle rect = (Rectangle)SelectionBoxItem;
    rect.Width = _clonedElement.RenderSize.Width;
    rect.Height = _clonedElement.RenderSize.Height;

    VisualBrush visualBrush = (VisualBrush)rect.Fill; // (x)
    visualBrush.Viewbox = new Rect(_clonedElement.RenderSize);
    visualBrush.Viewport = new Rect(_clonedElement.RenderSize);
}