NOTE: Under normal installation heights of 14-20" and moderately sloped driveways hold down straps normally will not be needed, but If your driveway is over 15 degrees or installations are over 20" tall Hold down straps will ensure the cans handle to stay in place.  
 
6" stretching ball bungee is a quick mounting retainer strap - use one per can.  It installs in seconds and can stay on the tow unit if not needed.