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'
);
}
}