I figured this out — hope it helps someone.
1. Copy single.php to your child theme
2. Rename it single-wpdmpro.php
3. Add fullwidth-content-area class
4. Remove the line that calls the sidebar.
Here’s the code.
<?php
/**
* The template for displaying all single posts.
*
* @package Poseidon
*/
get_header(); ?>
<section id="primary" class="fullwidth-content-area content-area">
<main id="main" class="site-main" role="main">
<?php while ( have_posts() ) : the_post();
get_template_part( 'template-parts/content', 'single' );
poseidon_related_posts();
comments_template();
endwhile; ?>
</main><!-- #main -->
</section><!-- #primary -->
<?php get_footer(); ?>
Forgot to mention – replace wpdmpro with the name of your custom post type (example is for WordPress Download Manager).