Position right sidebar metabox right below the publish metabox?

Still if its something like client requirement then below is somewhat hack sort of solution. Add this code to your add_meta_box() function. For the sake of understanding, below I have provided complete function but you will need to use selective code 🙂 Do let me know how it goes.

function myplugin_add_meta_box() {

    $screens = array( 'post', 'page' );

    foreach ( $screens as $screen ) {

        add_meta_box(
            'testdiv',
            __( 'My Post Section Title', 'myplugin_textdomain' ),
            'myplugin_meta_box_callback',
            $screen,
            'side'
        );
    }
    /* Get the user details to find user id for whom this order should be shown. Ideally, I believe it will be admin user. Make sure you change the email id*/
    $user = get_user_by( 'email', '[email protected]' ); 
    $order = get_user_option("meta-box-order_post", $user->ID);

    $current_order = array(); 
    $current_order = explode(",",$order['side']);

    for($i=0 ; $i <= count ($current_order) ; $i++){
        $temp = $current_order[$i];
        if ( $current_order[$i] == 'testdiv' && $i != 1) {
            $current_order[$i] = $current_order[1];         
            $current_order[1] = $temp;          
        }
    }

    $order['side'] = implode(",",$current_order);

    update_user_option($user->ID, "meta-box-order_page", $order, true);
    update_user_option($user->ID, "meta-box-order_post", $order, true);

}
add_action( 'add_meta_boxes', 'myplugin_add_meta_box', 2 );