In South Africa they don't even get excited about shooting people, nobody is going to worry about seagulls, Marpol or no.
I've seen boats using elastic about 10mm wide like one would sew into the top of your knickers. Span it not too tight but tight enough not to sag. With even a mild breeze it sets up a low hum that is just about inaudible (to me anyway) and it seems to work. The boats using it are noticeably more turd-resistant.