The best after market purchase I have made to date for my bike is a super loud air horn.
Scenario: Driving on the interstate and an 18 wheeler is either tail-gating you or trying to get in my lane with me (while looking directly at me knowing I had no place to go).
Solution: Press and hold this super loud air horn until the situation is resolved. Don’t speed up, don’t slow down. These drivers know that they are not acting right and when you bring attention to it they back off. I have had to hold the horn for only 30 or 45 seconds at the most (which is a pretty long time to continuously honk a horn) and this has never failed to work. If you are lucky, you may even get the attention of a highway patrolman as well.
Troy of Texas