duplicate featured image functionality in custom meta box

you can try this to add custom content in the metabox :

add_action("add_meta_boxes", function (string $post_type, \WP_Post $post) {

    // the post type can be tested here


    $GLOBALS["wp_meta_boxes"]["page"]["side"]["low"]["postimagediv"]["callback"] = function ($post) {

        // the original content to select the featured image
        post_thumbnail_meta_box($post);


        ?>

            <p>
                custom content
            </p>

        <?php

    };


}, 10, 2);