Courgette seed saving

The problem will be, if you leave it too late, there won't be enough time for it to ripen fully and you won't get any viable seed. If you have a few plants, it shouldn't matter that you don't get too many from the one you are saving seed from. It needs to grow into a marrow and the skin get hard like a squash. You may need a saw to get into it when it is fully ripe.