I use one of these, especially since my helmet doesn’t have d rings. I just run it through the helmet lock thingy that comes with the bike and throw the key in the side case.
Master Lock 8418D 6ft (1.8m) Long x Diameter Python™ Adjustable Locking Cable; and Black 5/16in (8mm) Wide
The Master Lock No. 8418D Python™ Adjustable Locking Cable 6ft (1.8m) long and features 5/16in (8mm) diameter braided steel for maximum strength and flexibility. The protective vinyl coating protects against scratches. The integrated pin tumbler keyed locking mechanism provides superior pick...