How can I pull media:thumbnail from an RSS feed with SimplePie and fetch_feed()?

I have two separate WordPress installs – I will call one Site A, and the other Site B. I want to pull the feed from Site B onto Site A, using fetch_feed(). I also want to include a thumbnail image. WordPress doesn’t include a thumbnail in the feed by default, so I created a custom feed, that includes the following:

<?php if(get_the_post_thumbnail()): ?>
<media:thumbnail url="<?php $image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'feed-thumb'); echo $image[0]; ?>" />
<media:content url="<?php $image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'feed-thumb'); echo $image[0]; ?>" medium="image" />

This seems to work, and it returns something like the following inside each :

<media:thumbnail url="http://www.site-b.com/wp-content/uploads/2013/01/thumbnail.jpg" />
<media:content url="http://www.site-b.com/wp-content/uploads/2013/01/thumbnail.jpg" medium="image" />

Now, I go back to Site A, and try to pull this feed using fetch_feed():

<?php // Get RSS Feed(s)
                include_once(ABSPATH . WPINC . '/feed.php');

                // Get a SimplePie feed object from the specified feed source.
                $rss = fetch_feed('http://www.site-b.com/custom-feed/');
                if (!is_wp_error( $rss ) ) : // Checks that the object is created correctly 
                    // Figure out how many total items there are, but limit it to 2. 
                    $maxitems = $rss->get_item_quantity(2); 

                    // Build an array of all the items, starting with element 0 (first element).
                    $rss_items = $rss->get_items(0, $maxitems); 
                endif;

                if ($maxitems == 0) echo 'No items.';
                    else
                    // Loop through each feed item.
                    foreach ( $rss_items as $item ) : 

                            if ($enclosure = $item->get_enclosure())
                            {
                                echo '<img src="' . $enclosure->get_thumbnail() . '" class="feed-thumb" />';
                            }
                            ?>

                        <p><?php echo esc_html( $item->get_description() ); ?>
                        <a href="<?php echo esc_url( $item->get_permalink() ); ?>"
                        title="<?php echo esc_html( $item->get_title() ); ?>">Continue Reading</a></p>

                    <?php endforeach; ?>

Everything works, except for the thumbnail. The title, the permalink, and the description are all returned correctly. However, the thumbnail does not return the URL. So, I am just left with:

<img src="" class="feed-thumb" />

How can I return the URL for the thumbnail image?

Thanks for reading this far.