If you look around you can find the bosch type I'm talking about. Some of them have heavier terminals. Trouble with a horn relay they are not continuous duty, although I realize this is not exactly a "continuous duty" application. Also I have no idea what the current rating of horn relays are. I'd sooner gamble on a Mopar starter relay instead.
This here is the heavy duty setup I was referring to. Maybe you can get somewhere from this
