I installed one of these on the R side subframe.
http://www.motionpro.com/motorcycle/partno/11-0005/ I can hang the helmet from the lock directly. However, I picked up a small cable at a bicycle store that has a large loop on one end and a smaller loop on the other. I thread the cable through the chin bar then slip the little loop through the bigger loop then attach the small loop through the Motion Pro Lock.
The cable length is such that the helmet can sit on the seat, but not hit the ground if it gets knocked off. It sounds more complicated than it is and works very well.