Try the Wolo Bad Boy air horn.
Self-contained unit, relatively small and easy to install.
I installed one in my Honda Helix that had the meep meep horn too.
Even tucked in behind the radiator and inside the front cowl when I hit this one it sure gets driver's attention.
The sound file on the web site does not do the horn justice.
A quick search and it shows up at numerous auto parts stores.
Hope this helps some.
Chris in Virginia
2006 Sunseeker 3100SS/LTD
2008 Smart Passion Cabriolet TOAD
1986 Honda Helix (Original owner)