I don't carry a lock, but I've heard that 1" diameter steel balls in a snack pouch up front are a good windshield breaker.
I don't think I've ever had someone intentionally try to run me off the road, but I've had people try to pass oncoming cars with me there. It's happened twice, I go farther into the lane and sit full upright with my bright clothing and both times they've braked and dropped back in behind the car they were trying to pass.