To get more volume, you need to steer away from the small piezo buzzers like the stockers, and get yourself some air powered horns such as the Magnum Blasters, Stebel Nautilus, or the Wolo Bad Boy.
I put the Wolo's on my 07, had to remotely locate the compressor, and even then still had to dremel out a tiny piece of plastic just forward of the radiator, which as Wheaton says, is unseen.
If there is another option out there, it's obviously a very closely guarded secret, and despite some serious searching. I couldn't find one that would improve on stock.