Add Filter – Pass Variable (PHP < 5.3)

You could try something like the second option, but I think it would be overkill. Instead, just write your own custom analog to wp_trim_excerpt() (the function that applies the excerpt_length and excerpt_more filters to the excerpt), like so:

function custom_excerpt( $new_length = 20, $new_more="...", $strip = false ) {
    // Start with the content
    $text = get_the_content('');

    // Do stuff to it
    $text = strip_shortcodes( $text );
    $text = apply_filters('the_content', $text);
    $text = str_replace(']]>', ']]>', $text);

    // Use custom values
    $excerpt_length = $new_length;
    $excerpt_more = $new_more;
    $text = wp_trim_words( $text, $excerpt_length, $excerpt_more );

    // Strip?
    if ( ! $strip ) {
        $text="<p>" . $text . '</p>';
    }

    // Output
    echo $text;
}

Overkill method:

function custom_excerpt( $new_length = 20, $new_more="...", $strip = false ) {
    // Excerpt length
    add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );
    function custom_excerpt_length( $new_length ) {
        return $new_length;
    }

    // Excerpt More
    add_filter( 'excerpt_more', 'custom_excerpt_more' );
    function custom_excerpt_more( $new_more ) {
        return $new_more;
    }

    // Output
    $output = get_the_excerpt();
    $output = apply_filters('wptexturize', $output);
    $output = apply_filters('convert_chars', $output);
    if(!$strip)
        $output="<p>" . $output . '</p>';
    echo $output;
}