Figured it out! Here’s the get_posts method to add a widget (or any content into a masonry layout at the “$counts” number you define)
<div class="arrange">
<?php
$args = array(
'posts_per_page' => 10000,
'offset' => 5,
'orderby' => 'post_date',
'order' => 'DESC',
'exclude' => 'none',
// 'post_type' => array('post', 'page', 'video_type', 'review_type' ),
'post_status' => 'publish',
'suppress_filters' => true
);
$posts = get_posts($args);
$counts = 0 ;
foreach ($posts as $post) : setup_postdata($post); $counts++;
get_sidebar('corner-box');?>
<div class="box">
<a href="https://wordpress.stackexchange.com/questions/83824/<?php the_permalink() ?>">
<h2><?php the_title(); echo $counts; // to see your box number ?></h2>
<?php if (has_post_thumbnail()) {the_post_thumbnail('masonry');}
the_content();?>
</a>
</div><?php if($counts == 9) { get_sidebar('masonry');}
endforeach; ?>
</div> <!-- end masonry -->