I met a guy a couple of days ago who took an air horn off of an old truck and strapped it to his bike.
I connected a long rubber hose to it which he strapped to his handlebars.
He then had to SUCK hard on the hose to initiate the horn blast. It sure worked and gave a frightful blast, but looked and seemed ridiculous.
I am able to whistle loudly which seems to work well and is fast and convenient. On cold winter days, though, when my lips are cold, it is not reliable.