helmet holder
 

apparently you can lock a helmet to the pillion seat. How the hell does that work?! :?

You sort of put the straps into the loops at the front, then close the seat. I've never used it for riding around with a lid on the back but I've often done it just to leave my lid fairly securely, when shopping etc.

Also works under a cowl cover. I don't think I would use it when riding, looks like it would rub the paint on the bike and on the helmet. I also use it when shopping, but look out for people using your helmet for a rubbish bin. The hook should be apparent, just slip one of your D-rings over it and close the lid. I think there is a drawing in the manual, the dealer showed it to me when I was looking at the bike.