You know what Ray, a lot of these 12v airhorns I have seen have a bit of a delay and a lead in to full sound - as there is no air tank, you have to wait 0.5s for the pressure to build up. I wonder if you could use the air suspension
Or what about sythesised airhorn like the emergency services use?
I have got a spare able2 shome if you want it for nothing, but it needs a 100w sounder which is £36 on ebay/aliepress...
https://www.swps.com/30-2109.html