Hey guy! How are you doing? I don't know if you're still in doubt but I'm posting the way which I've done to fix this problem.
I've editted the code between the lines 198 and 210 on custom-post-type-list-widget.php file. When you change $out variable, you're changing your output content. Then you use the get_the_post_thumbnail() function. My code ended up being as folow:
$out.= "\r\n<div class=\"item-sugestao-leitura\">".get_the_post_thumbnail($ID, array(66,94))."$post_title
Where $ID is the id of my post, array(66,94) are the dimensions of the thumbnail and $livro_autor is a variable which receives a meta value:
Anywaw, keep in touch if you're still in doubt
Alan Hidalgo Pagoto
Curitiba/PR - Brazil