I didn't get one either, but I'll tell ya what I used.
i took a valve stem, the kind you pull into the hole in the rim.
Cut the stem part off, and used the round part as a "cushion".
Already predrilled for a bolt and ain't had no trouble out of it in about two years now.
Hello... I bought the horn, but it is no **** hot to install it now, if you are patient, give me several weeks & I will see if can part with it....sorry cant do anything sooner, its been around 117....