Add HTML to single post tag

I found the best solution was to add a custom field, head, and then build a short plugin to add the contents to the head tag:

add_action('wp_head', 'add_HTML_head');
function add_HTML_head(){
    global $post;
    if(!empty($post)){
        // get custom field for 'head'
        $headHTML = get_post_meta($post->ID, 'head', true);
        if(!empty($headHTML)){
            echo $headHTML;
        }
    }
}

I’ve packaged it up into a simple plugin.