plugin that includes external scripts?

I have a script I’m using which is a classified ads, and it contains many files all held in one folder called “jobs.” I tried using some php to call the script into my WP content area by changing the WP theme’s index.php file to read as such:

<?php get_header(); ?>
    <div id="wrapper">
        <div id="content">
            <?php
                require_once ($_GET['filename']);
            ?>
        </div>
        <?php get_sidebar(); ?>
    </div>
<?php get_footer(); ?>

and if you visit this url, you can see it has loaded into the page:

https://tronicscribe.com/testies/index.php?filename=jobs/index.php

however there are many fatal errors and pages not found when you click on the categories and listings. plus I would prefer to keep my normal index.php in the theme so that it still includes my blog posts, and then have a different page load the script for the classifieds.

so I’m wondering if before I try to tackle all this, is there a plugin that would do the same thing? such as allow me to include files from a different directory such as my “jobs” folder that has the script? if so, I’d greatly prefer to use a plugin to manage this. thanks!