I built a pair of 6 foot long PVC air horns a while back and they sounded pretty insane. It was never originally intended to be installed on a vehicle but I have seriously been considering putting one of them on my truck. Now for what it's worth it would be a nightmare to try to install something like this on a vehicle without a nice long straight empty frame rail but the concept could easily be scalled down too. The nice thing about it is it was extremely cheap to build, somewhere around $20. But you still would have to come up with some kind of air management to make it function.
I was just considering installing a 5 gallon air tank (which I already have) and filling it with shop air. I don't use my horn that much so I think this setup would work out alright for me. I would probably add a gauge inside the cab so I can keep an eye on the tank pressure and I would also keep my current small direct compressor driven air horns as a back up. I might even install a 3 way toggle so I can use either set as the situation warrents.
Here is a pic of the ones that I made hooked up to an old fire extinguisher as an air source.
They are not quite train horn loud, but they are very low pitched. They sounded kind of like a pissed off battleship. I made these originally to ring in the new year last year. And the 2.5 gallon tank regulated to 50 PSI ran the horns for almost a full minute.
And here is a little diagram that I made that crudely illustrates how they were assembled.
