Hey charlie d, It won't sound authentic on 12v might as well get a 12v horn and save your original. The crowd on this site is usually restoring/operating cars in orthodox form. Might have better luck on sister site "H.a.m.b" for customizing your Model A.
Still, Lot of good detailed info on this site. There are a lot of gurus lurking around here that have helped me in the past. These are also the folks who strive to have their cars exactly how it left the factory all those years ago. Really an admirable facet of Model A culture!