After a painful and futile four hours last month of trying to install a replacement air horn thru the fender well, I gave up. It was agony trying to work in the cramped space, but after I got it in place finally, I was unable to get the horn mounted vertically. The closest I could come was about 30 degrees off vertical.

My understanding is this will 'kill' the horn eventually.
I retreated and last week I followed
Evilution.co.uk instructions and pulled the front of the car off. Only took a hour and it was all removed. Installed the horn on the original bracket (in front of it) using a modified 3" "L" reinforcing bracket I picked up at Lowe's. Cut one leg of the "L" off short - just past the first hole - and bolted (1/4 X 1") it to the hole on top of the horn's mount. Long leg of the "L" got the outer hole enlarged enough to accept a 5/16th inch X 3/4 inch bolt with locking nut. That mounted to the original bracket in the same area as the stock horn. Tight fit, so had to leave the 5/16th loose until everything was in place. Set it up vertically and then tightened that down.
Another 45 minutes for getting the front back on and I was done. About 3 hours total counting fabricating the "L" bracket and lunch. And I think in the future I could get the front off in about 30 minutes, even working by myself as I was.
Love the new sound~! The new horn is a Stebel Nautilus.