Its a real tragedy Marty and a huge loss for the family especially if they were at the scene our hearts go out to them. I read in one of the media reports that it may have been a student and instructor in the Cessna. ( media is always right )
"Old Koreelah, a horn could be mounted forward facing in the cowl with a flap when closed would still form the
cowl shape, activated from the power supply to the horn through a model aircraft servo, light and easy as. I wouldn't be too concerned by getting sound pitch levels right to suit different hearing types, just get it as loud as possible. Some will hear it which in turn would alarm others.