How to add metabox ONLY to specific WooCommerce product type [closed]

You should be able to get the current product id using the using the $post global variable and use that with the WooCommerce function get_product() to the the product object and test its product type.

public function __construct() {
    add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );
}

public function add_meta_box( $post_type ) {
    $post_types = array('product');     //limit meta box to certain post types
    global $post;
    $product = get_product( $post->ID );
    if ( in_array( $post_type, $post_types ) && ($product->product_type == 'simple' ) ) {
        add_meta_box(
            'wf_child_letters'
            ,__( 'Picture Preview', 'woocommerce' )
            ,array( $this, 'render_meta_box_content' )
            ,$post_type
            ,'advanced'
            ,'high'
        );
    }
}