I made my own battery box with a key to lock out the ignition. No key, no start.
I also made a clutch cable pulley and I can slip the cable off the pulley, preventing the back wheel from turning, so the would be thief can't pedal or walk off with my bike. Squeezing the clutch handle at that point does nothing.
A good cable & lock helps, but why make it any easy for them?
I've also been considering these products.
Siren lock to deter thieves.
GPS tracker for the thief who will not be detered. They offer 3 models: one that hides in the handlebar headstem, one for the seat post, and one as a rear tail light. What is real nice about this is you can download an app for your phone to map where it is, where it has been, track your mileage, & top speed.
My bike is also legally registered & tagged, mostly so cops won't jack with me anymore, but also for identification in the event it is stolen.