http://www.cruisercustomizing.com/detail.cfm?Category_ID=110&manufacturer_ID=16&product_ID=20018&sblid_name=Kuryakyn_License_Plate_Helmet_Lock_w_Mount___Universal&model_id=0 Add this license plate helmet lock and conveniently lock your helmet to your motorcycle. The simple little mounting plate easily bolts to the back of almost any motorcycle and allows the easiest access for locking your helmet. Fits under all standard motorcycle license plates.
It was easy to modify this BMW Motorcycle Helmet Lock for use on my Harley. Mounts behind my license plate. There is a helmet lock on both sides of my license plate. It cost $50 is by Helmet Guardian.