Add this code to the Theme’s function.php file to add some custom HTML inside the loop items in your single wishlist page.
add_action('wishlist_single_loop', 'wishlist_single_loop_20201908', 5, 2);
function wishlist_single_loop_20201908($item_id, $wishlist_id){
//echo '<pre>'.var_export($wishlist_id, true).'</pre>';
?>
<span>Custom HTML here</span>
<?php
}