Just a suggestion, forget the custom cushions and get some "Comfy Seats". But don't buy cheap imitations if you go down this road. Cheap ones have crap covers, inferior foam and you get to feel the frame when you are sitting in them. But the ones in the link below a great. No more gunnel bum.
comfortseat.com.au/main/index.htmlI got mine (on special) from the Sanctuary Cove Boat show, but I would imagine they would be available at the Sydney Boat show as well.