I've read the manual about
customizing 404 error pages.
However, I would like my 404 error page to be
like (not the
same as) a static page, where my custom 404 error message is within the skin, rather than an empty page like it is for the /xcart/skin/common_files/404/404_en.html page. That would be cool!

Also, I do NOT want to do a redirect from the 404 error page either with a meta refresh or a 301 redirect. (Google doesn't like those. Google it.) I don't think I want to use some iframe

either.
The html files for XC static pages (example: /public_html/skin/common_files/pages/en/privacy_statement.html) don't include the necessary includes to display the containing skin/page structure. They are "called" from its parent php file
somewhere. Perhaps if I knew where the parent php file was, I could clone and modify it and put it in the /xcart/skin/common_files/404/ folder and it would accomplish this task?
TIA!