I use a plastic-coated, all weather MasterLock with long hoop portion to go around my luggage rack and through the D-rings. Works fine, hardened steel. Won't stop anyone from cutting the straps but few things will. If I had an expensive helmet like an Arai I would get this:
http://www.pacsafe.com/www/index.php?_room=3&_action=detail&id=27
