I just use a lock I use for my snow skis.
I feed the cable through the jacket arm as well as the helmet, then either around the handle bars or sissy bar. Don't need to carry anything around.
I just use a Lockwood padlock with a long u shank , if i have 2 helmets one on pillion seat and other on rear rack and i lock the d rings together thru the sissy bar . If i only have my helmet than i lock it to the rack with same lock
As Mittzy or I try and travel light enough that the trunk is empty - can fit 2 full face helmets in there if needed; as with albertaduke my helmets don't have D-rings.
I have Kury grips so I installed the Kewl Locks on each side. They function as locks as well as bar-end weights.
They've worked quite well as helmet locks over the past 5 years (already??) but I recently purchased a Nolan helmet which doesn't use D-rings, thus it won't work with the Kewl Locks.
I just leave my helmet on the sissy bar
99% of people figure it's not with the beating for a three hundred dollar helmet
That in Minnesota touching a motorcycle that is not yours is a felony
And a sure way to get your ass beat