404 with permalinks - and prefix URL.

Hi guys.

I've built a custom 404 page and trying to display it while the page is still staged.  I've edited permalinks.php to read

$GLOBALS['PERMALINKS']['404_not_found_filepath'] = __DIR__ .'/404/'; // filepath to include if Permalink isn't found

changed to various versions of than 

$GLOBALS['PERMALINKS']['404_not_found_filepath'] = __DIR__ .PREFIX_URL. '/404/'; // filepath to include if Permalink isn't found

Any suggestions?  If a problem I can move that file to the permalinks folder, but Id rather keep it in the root.

I currently get the error: Notice: Couldn't find filepath for 404 page: '/home/stmarysbundaberg/public_html/cmsb/plugins/permalinks/~stmarysbundaberg/404/'! in /home/stmarysbundaberg/public_html/cmsb/plugins/permalinks/permalinks.php on line 279