Add shark fin to car roof legally?

I'm thinking of adding a fibergl fin to the top of my car. Does anybody know if this is legal/illegal? My only real reason for doing it is I have a tiny silver car in an area with a lot of big trucks and other tiny silver cars, and can never find where I'm parked. I don't have a clicker to make it beep so that doesn't work either. Right now I'm just curious as to the legality of bolting an 18" tall fibergl fin to the top of my car. I live in PA if that makes a difference.