I used to lock my helmet together with my bike with a U-lock. Now that I start using TiGr Lock, I find that the lock arms are usually covered with muck because of its lower locking position, and I don’t want my helmet straps get dirty.
I recently found this little luggage lock called “Lewis N. Clark Tsa Lockdown Triple Security Cable Lock”: . The longer cable on it is just enough to route through both rails on the saddle, and the shorter one is great for locking the helmet. This might be an option for securing your helmet.