WE HAVE NOT YET BEGUN TO HONK
Do you self-identify as a Freedom Trucker - but drive a Honda Civic?
Well, DIY'ers have been making boat and train horns for decades using commonly available PVC and a variety of spare parts… and many of them have been posting how-to videos on YouTube. And, as you expect from people who would build an air horn, many of them also like to mount them on their cars.
Check it out - https://www.youtube.com/results?search_query=diy+boat+horn
I like this one - https://www.youtube.com/watch?v=iIoWuCM0IHo