Creating a custom page in wordpress from a template part with no wordpress head(), foot() or theme

custom-path.php

<?php

/**
 * Class CustomPage_Frontend
 */
class CustomPage_Frontend {

    private static $instance = null;

    public $path = 'custompage';

    /**
     * Just to be able to call it statically.
     */
    public static function init()
    {
        if (null == self::$instance) {
            self::$instance = new self;
        }

        return self::$instance;
    }

    /**
     * Boot this thing up
     */
    public function __construct()
    {

        add_action('init', array($this, 'rewrite_rules'));
        add_action('template_redirect', array($this, 'template_redirect'));

        add_filter( 'query_vars', array($this, 'query_vars'));
    }

    /**
     * Rewrites
     * @return Void
     */
    public function rewrite_rules()
    {
        add_rewrite_rule(
            '^'.$this->path.'/?$',
            'index.php?'.$this->path.'=yes',
            'top'
        );
    }

    /**
     * Query vars
     * @param  Array $vars
     * @return Void
     */
    public function query_vars( $vars )
    {
        $vars[] = $this->path;
        return $vars;
    }


    /**
     * Templates redirects
     * @return Void
     */
    public function template_redirect()
    {

        if (get_query_var($this->path) == 'yes') {
            add_filter( 'template_include', function() {

                get_template_part('templates/custom-page');
            });
        }
    }



}

CustomPage_Frontend::init();