Load custom template with template_include / locate_template

Hi,

I want to load a custom template and found this example in codex:
http://codex.wordpress.org/Plugin_API/Filter_Reference/template_include

This example includes a new template on a page called ‘portfolio’ if the new template file was found.

add_filter( 'template_include', 'portfolio_page_template', 99 );

function portfolio_page_template( $template ) {

	if ( is_page( 'portfolio' )  ) {
		$new_template = locate_template( array( 'portfolio-page-template.php' ) );
		if ( '' != $new_template ) {
			return $new_template ;
		}
	}

	return $template;
}

I want to load a custom template from a plugin (so not from theme), so I should adjust this part I guess:

$new_template = locate_template( array( 'portfolio-page-template.php' ) );

Who can tell me how to load a custom template from the plugin directory?

Guido