Select field on feature image metabox

/**
 * Select field on featured image
 *
 */
add_filter( 'admin_post_thumbnail_html', function ( $content, $post_id ) {
    $field_opt   = '';
    $field_id    = '_content_bg_position';
    $field_value = esc_attr( get_post_meta( $post_id, $field_id, true ) );
    $field_text  = __('Background Position');
    $field_var   = array('', 'center', 'top', 'bottom');

    var_dump($field_value);

    foreach ($field_var as $item)
        $field_opt .= sprintf('<option value="%1$s"%2$s>%1$s</option>', $item, selected( $field_value, $item, false));

    $field_label = sprintf(
        '<p><label for="%1$s">%2$s</label><br /><select name="%1$s" id="%1$s">%3$s</select></p>',
        $field_id, $field_text, $field_opt
    );

    return $content .= $field_label;
}, 10, 2 );

add_action( 'save_post', function ( $post_ID, $post, $update ) {
    $field_id    = '_content_bg_position';
    $field_value = isset( $_REQUEST[ $field_id ] ) ? $_REQUEST[ $field_id ] : 0;

    update_post_meta( $post_ID, $field_id, $field_value );
}, 10, 3 );