Add div to specific sub-menu

I researched in google, yandex but had no success.

With this code I can set a div to sub-menu but this code add to all sub-menus, I just need to add div to specific sub-menu.

class Megratron_div extends Walker_Nav_Menu
{
    function start_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("t", $depth);
        $output .= "n$indent<div id='megatron'><ul class='sub-menu'>n";
    }
    function end_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("t", $depth);
        $output .= "$indent</ul></div>n";
    }
}

¿How can I do that?

I really appreciate your support.