I can recommend this horn type for 1/5 & 1/4 scale. It is easy to adjust to even width and robust in use. As Overbored77 recommended - I either make thin plywood cheeks around the base before covering or set it in a 1/2" dowel epoxied in the rudder for ARFs.
http://www3.towerhobbies.com/cgi-bin...?&I=LXBWL4&P=7