Interested as well.

Good idea on the a-oo-ga horn, it is a little big compared to a normal horn but this sound is awesome
Just found one that looks like a good choice, 110 DB, 12V and less than 5 amp draw and has a nice look

this one around US$50
http://www.buy.com/pr/product.aspx?sku=229381681&sellerid=36551499

and the HB version, not so loud 100DB and with a different "red plastic" look and for US$12
http://www.harborfreight.com/100-db-old-fashioned-sound-ooga-air-horn-96291.h...

In the past I have done conversions on car, from the tiny beep to the ones that came in on BMWs, it is a dual tone system, dual horns actually, one deep tone and the other one is not so deep, both activated at same time make a good sound that definitely calls for attention

Horns are interesting.   There have been a bunch of psychological studies on them and how and why they arouse attention.   I think this is why the beemer horns are two tones that are chosen not to be harmonics of each other.   Jet fighter aircraft have female voice warning systems.   If you're married you fully understand this.

A dual horn setup actually the most effective. A low one and and high one that are out of key with each other. When things are out of key, they instantly draw attention. Think of being at a concert or even at karaoke night - and someone hits the wrong note. Everyone cringes. You could be mid-sentence talking with a friend, not even focused on the music, but that wrong note just makes your whole body convulse almost instinctively.

This is why train horns sound the way they do. Train horns are only loud so you can hear them from very far away - it has nothing to do with what emotions they invoke. Their off-key NON-harmonization makes you take notice of them regardless of what you may be doing.

My point? Two small off-key horns are better than one large loud horn.