I mow a widow lady's lawn with her JD 345 and had the same thing happen. I'd cleaned the screen on top, put the hood down as normal and "plunk" I heard something hit the floor. It was the bottom corner of the hood (picture). I asked the dealer if this is a common problem, he said "people run into things all the time". Well, I've never run into anything. I suspect since the exhaust exits nearby it might make the plastic brittle? Holy Cow! Are you saying that little piece is $300!
I'm going back to the dealer and have a talk!