+1 on the F horn, that's the right one..
Make sure the teeth are lined up correctly. I used a tiny little bit of WD40 on the teeth of the horn itself (not the servo!) and it slid on like buttah. If the horn doesn't line up at 90 degrees, it's better to have it point slighty toward the center of the car, not away - good tip from a local pro.
The alternative is you could pick up a clamping aluminum horn, I believe Racers Edge makes them, and those slide on with zero resistance. They have two screws on either side that you tighten up to clamp onto the spline once you've got it lined up correctly. That's what I'm using now, and it's never going anywhere.