Loop within a Relationship-Field?

Okay, here’s what i got:

1. A Page called “Downloads”
…with an area on the right side where i want to place different donwload-boxes related to the page content.

2. A Custom Post Type called “Download-Boxes”
…every Post is a different Download-Box with the possibility to place some Donwload-Items with a Loop-Field (see number 4.)

3. A Custom Field Group called “Box”
…this is the Relationship-Field which appears on pages only but is limited to the post types “Download-Boxes”.

4. A Custom Field Group called “Loop”
…this appears on the post type “Download-Boxes” only. It has two fileds in the loop: “uploadurl” and “filename”

Is there a way to output the loop like i do it with the standard values like “post_title” or “post_content”?

In other words:

I want this:

<?php
$fields = CFS()->get('loop');
foreach ($fields as $field) {
echo '<a href="'.$field['uploadurl'].'">'.$field['filename'].'</a>';
}
?>

into this:

<?php
$values = CFS()->get('box');
foreach ($values as $post_id) {
$the_post = get_post($post_id);
echo '<div class="sidebaritem">';
echo '<h3>Downloads</h3>';
echo '<div class="widgettext">';
echo '</div>';
echo '</div>';
}
?>

https://wordpress.org/plugins/custom-field-suite/