How to create a dynamic page based on form data with a plugin?

I have a form that users fill out. When they finish, they are redirected to a 3rd party site to set up payment options. After completing the payment process, they are sent back to my site with a variety of $_POST variables.

My plugin needs to create a thank you page based on those POST variables. It’s a simple little page with a title and a few paragraphs of text.

I do not want these pages to be stored in the database. They will be unique to each transaction.

I attached a function to init that parses the $_POST data (after sanitizing, of course.) That function creates two global variables: $my_title and $my_content I’m not sure how to turn those into a page for the user, though.

The payment system will send users to www.mysite.com/thank-you

So far, I have a filter on rewrite_rules_array to add my url:

function my_rewrite_rules( $rewrite_rules) {
    $rule = array('thank-you/?$' => 'index.php?formresult=thank-you');
    $rewrite_rules = $rule + $rewrite_rules;
    return $rewrite_rules
}

I filter query_vars to add mine:

function my_insert_qv($vars) {
    array_push($vars, 'formresult');
    return $vars;
}

Then, I have an action on template_redirect where I think the action should be. Maybe this is the wrong path.

function my_template_redirect() {
    gloval $wp;
    $qvs = $wp->query_vars;
    if (array_key_exists( 'laundry', $qvs && $qvs['laundry'] == 'thank-you' ) {
        global $wp_query;
        $wp_query->set( 'is_404', false );
        // ... now what?
    }
}

How can I show the user a page that loads the default page template from the active theme and shows my dynamic title and content?

Something like this should work, though I’m not sure how meta, title, etc.. will behave, you’ll want to test everything thoroughly!

function my_template_redirect() {
    global $wp;
    $qvs = $wp->query_vars;
    if (array_key_exists( 'laundry', $qvs && $qvs['laundry'] == 'thank-you' ) {
        global $wp_query;
        $wp_query->set( 'is_404', false );
        // ... now what?

        $post = new stdClass();
        $post->ID= -99; // fake ID, hehe
        $post->post_content="some content";
        $post->post_excerpt="an excerpt";
        $post->post_status="publish";
        $post->post_title="My fake page";
        $post->post_type="page";
        $wp_query->queried_object = $post;
        $wp_query->post = $post;
        $wp_query->found_posts = 1;
        $wp_query->post_count = 1;
        $wp_query->max_num_pages = 1;
        $wp_query->is_page = 1;
        $wp_query->is_404 = false;
        $wp_query->posts = array($post);
        $wp_query->page = 1;
        $wp_query->is_post = false;

    }
}