I've been tempted to get one of those handlebar-mounted car horns for use when cars cut me off or nearly run me over.

I have a bell on my bike but I actually don't use it that much. I don't often encounter situations where I need to pass a person that is occupying the width of a trail and if I do then usually a "Passing on your left/right!" will suffice.

https://www.instructables.com/The-Train-Horn-Bicycle/

That’s a monstrosity that I can get behind. I just need some to engineer it to be more compact

Could try a high volume air pump, but those only seem to do about 5 psi.

With a limited number of uses you could try CO2 cartridges or use blanks to fill a smaller compression chamber.

Noise cancelling headphones need to be forbidden when participating in traffic. I'm so sick of pedestrians not seeing because they're staring at their phone and not hearing because of headphones, essentially sleepwalking through the streets.