What I use is a Ohmite RESISTOR, 2 OHM, 100 WATT, ADJ TUBULAR, WIREWOUND, +-10% TOLERANCE.The Galco part number is D100K2ro.
It is a adjustable resister so you get adjust what sound you want from your horn. I found it in a old Restorer article? You have to make a simple bracket that mount on the...