I used short screws that fit tightly into the plastic of the horn; they were in the packet of hardware.
I didn't use any washers at all between the ball and the horn - the tie rod end moved freely on the ball. But now that I think about it, a washer on the screw side to keep the tie rod end over the ball is probably not a bad idea. When I assembled it, it looked like one side of the rod end was slightly smaller than the other, so that is the side I put towards the horn. The larger opening faces the screw's head.
I'd thought about doing some photos during assembly since I have all the equipment for making nice product shots but I was too excited and impatient to get it built to stop and set all the lighting up to do it... *sigh*