Here at GHYC we have an automatic horn controlled from the app (starttimerapp.com) on the phone, that also gives voice cues on what actions need to be done and countdown. I also made a battery-powered box for the horn based on the same automatic controller.
As for the horn itself - those automotive ones above seem to be a good value. Pure electric ones are not very loud.
Disclaimer: I make both the app and the horn controller.
P.S. the app is only released for iOS, but the Android version is available in beta. Contact me if you want to give it a go.
