mk_custom_admin_order_data_after_order_details.php

<?php

function mk_custom_admin_order_data_after_order_details($order) {
        ?>
        <div class="order_data_column">
            <h4><?php _e('Custom Field'); ?></h4>
            <?php
            echo '<p>Custom Value</p>';
            ?>
        </div>
        <?php
}

add_action('woocommerce_admin_order_data_after_order_details', 'mk_custom_admin_order_data_after_order_details');