Helmet Lock

Yup:
https://www.motone.co.uk/motorcycle...t-side-shock-mount-helmet-lock-and-keys-p1581

 

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

 

Check out www.lidlox.com

 

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.