I solved this by using the following module & some code:

http://drupal.org/project/panels_extra_styles

Panels region > Change style > Wrapper: raw.

Content prefix:

<script type="text/javascript" src="/misc/form.js"></script> 

<script type="text/javascript" src="/misc/collapse.js"></script> 

<form>
  <div>
    <fieldset class="collapsible form-wrapper">
      <legend>
        <span class="fieldset-legend">Attachments</span>
      </legend>
        <div class="fieldset-wrapper">

Content suffix:

        </div>
    </fieldset>
  </div>
</form>

Save and update. Voila!

References: Supplied if needed.