Thumbnails on next/previous links in custom post type single.php

I’ve been successful in using the following code to display thumbnails for next & previous links for normal posts but this does not work at all for custom post types.

<div id="cooler-nav" class="navigation">
                <?php $prevPost = get_previous_post(true);
                if($prevPost) {?>
                <div class="nav-box previous">
                    <?php $prevthumbnail = get_the_post_thumbnail($prevPost->ID, 'tiny_thumb' );?>
                    <?php previous_post_link('%link',"$prevthumbnail  <p>%title</p>", TRUE); ?>
                </div>

                <?php } $nextPost = get_next_post(true);
                if($nextPost) { ?>
                <div class="nav-box next">
                    <?php $nextthumbnail = get_the_post_thumbnail($nextPost->ID, 'tiny_thumb' ); } ?>
                    <?php next_post_link('%link',"$nextthumbnail  <p>%title</p>", TRUE); ?>
                </div>
                <?php  ?>
            </div><!--#cooler-nav div -->

I’m guessing it must be a special query to look for my custom post type “portfolio”

Any help would be greatly appreciated

EDIT

<nav class="clearer">

                <?php if( $prev_post = get_previous_post() ): ?>
                    <?php echo $prev_post->ID ?>
                <div class="nav-box previous">
                    <?php $prevthumbnail = get_the_post_thumbnail($prev_post->ID, 'tiny_thumb' );?>
                    <?php previous_post_link('%link',"$prevthumbnail  <p>%title</p>", TRUE); ?>
                </div>
                <?php endif; ?>

                <?php if( $next_post = get_next_post() ): ?>
                    <?php echo $next_post->ID ?>
                    <div class="nav-box next">
                        <?php $nextthumbnail = get_the_post_thumbnail($next_post->ID, 'tiny_thumb' );  ?>
                        <?php next_post_link('%link',"$nextthumbnail  <p>%title</p>", TRUE); ?>
                    </div>
                <?php endif; ?>

            </nav>

Other loop not working (I am getting thumbnails & links but no exclusion of current post or randomizing of order)

<?php   
                    $t_args = array (
                       'tax_query' => array (
                          array (
                            'orderby' => 'rand',
                            'post_type' => 'portfolio',
                            'taxonomy' => 'service',
                            'field' => 'slug',
                            'terms' => $service_type,
                            'post__not_in' => array($post->ID)

                          )
                       )
                    );

                    $query = new WP_Query( $t_args );

                    if( $query->have_posts() ) {
                       while ( $query->have_posts() ) : $query->the_post();

                          // output your stuff
                          echo '<div class="more_from_thumb"><a href="';
                          the_permalink();
                          echo '">';
                          the_post_thumbnail();
                          echo '</a></div>';
                       endwhile;
                       wp_reset_query();
                    } ?>