A Lightweight saddle bag with a closed loop seat-post tether.
Getting gear and parts stolen from your bike is almost inevitable. Whether it happens after locking your bike up to get a coffee or stepping into a shop to pick up an energy bar, stuff gets stolen. Saddle bags can typically be taken in a matter of seconds often containing spare cash or a credit card for emergencies, and necessary spare tube and tire levers.
By making a saddle bag harder to remove from the bike it can act as a deterrent or at least slow down a would be heister during the few moments that matter most.
Why spoil a ride when you could just be better prepared. Ride secure ride strong.