function modRSS($content) {
   global $post;

   if ( has_post_thumbnail( $post->ID ) ){
      $content = '<div>' . get_the_post_thumbnail( $post->ID, 'full', array( 'style' => 'margin-bottom: 15px;' ) ) . '</div>' . $content;
   }

   $content = $content . '<a href="' . getCustomField('Affiliate Link') . '">Peek</a>';

   return $content;
}

add_filter('the_excerpt_rss', 'modRSS');
add_filter('the_content_feed', 'modRSS');