Adjusted file path function from the cache-enabler plugin.

site-base-cache.php

/**
* Generate the Cache Path
*
* @param string $path
*
* @return string
*/
function _ce_file_path($path = NULL) {

  $url = parse_url( $_SERVER['REQUEST_URI'] );

  $page = isset( $url['path'] ) ? $url['path'] : '';

  $path = sprintf( '%s/%s', SITE_CACHE_PATH, $page );

  if ( is_file($path) > 0 ) {
      header( $_SERVER['SERVER_PROTOCOL'] . ' 404 Not Found', true, 404 );
      exit;
  }

  // add trailing slash
  $path = rtrim( $path, '/\\' ) . '/';

  //remove double slashes
  $path = str_replace( '//', '/', $path );

  //remove double /a/a
  $path = str_replace( '/a/a', '/a', $path );

  return $path;
}

    /**
     * Cache Path
     *
     * Adjusted file path generator. Slightly different from the one
     * in advanced-cache.php
     *
     * @since   1.0.0
     * @change  1.4.0
     *
     * @param   string  $path  uri or permlink
     * @return  string  $diff  path to cached asset
     */
     private static function _file_path($path = NULL) {
       $url = parse_url( $_SERVER['REQUEST_URI'] );

       $page = isset( $url['path'] ) ? $url['path'] : '';

       $path = sprintf( '%s/%s', SITE_CACHE_PATH, $page );

       if ( is_file($path) > 0 ) {
           header( $_SERVER['SERVER_PROTOCOL'] . ' 404 Not Found', true, 404 );
           exit;
       }

       //remove double slashes
       $path = str_replace( '//', '/', $path );

       //remove double /a/a
       $path = str_replace( '/a/a', '/a', $path );

       if ( is_file($path) > 0 ) {
           wp_die('Path is not valid.');
       }

       // add trailing slash
       $path = rtrim( $path, '/\\' ) . '/';

       return $path;
    }