These guys might have a duckbill thatll work?
http://www.customgasketmfg.com/Duckb...a-RubberValves
Saw these the other day and thought they were pretty slick. Dunno howd they do backing up though on this particular boat they were on though... Real thin plastic, couldnt tell what material exactly. Slapped shut with no effort. Maybe a vertical orientation on the transom?
Untitled by
dirtwheelsfl, on Flickr