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