Simple, just get a cable lock, run it thru the frame and thru the helmet. It also has other uses. https://www.zoro.com/master-lock-lo...ZyyM8Ve0lunlg9enhyqXP_AsTlB6avZoaAi5KEALw_wcB
Couple of suggestions from Oxford Products. I've got the helmet bag thing, looks good, not used it yet but should stop my helmet filling up with rain. https://www.oxfordproducts.com/motorcycle/brands/oxford/helmet_accessories/lockable_helmet_bag/ https://www.oxfordproducts.com//motorcycle/brands/oxford/helmet_accessories/lidlock_black/ Also in my Owner's Handbook page 70, it says that Street Twins, T100s and T120s have a helmet hook on the left under the seat.
