Sorry for my language ....
If you use e a variable that marks a completely different code in plug.inc.php
not used by default plug.tpl, and is used popup.tpl in the skin.
What works can look in the code cotonti, in a file plug.inc.php from line 139
We'll see one hundred instead of $ plugin_title, $ plugin_body used
$ popup_header1, $ popup_header2, $ popup_footer, $ popup_body,
but do not use a common header and footer.
View attentively and plug.inc.php, popup.tpl and everything will become clear ...