I have a Thunderbird horn fitted on the same mount as the stock. Price was ~$35 and it is definitely louder than the stock. No fiddling around with a relay, fabbing special brackets, splicing, or running extra wiring.

Blue/White 2007 TBA, Thruxton needles, Unifilter, AI removed, Polaris Bellmouth, Bubs, Nology Coils/wires, Lightbar, Ricor Intiminators, Hagon Nitros, Tall Sissy Bar w/luggage rack, Dart flyscreen & Lowers. 130 Mains, TrueGel Battery MG12-BS.