I fail to see why you would put a doughnut filler in, except to make a domed horn more flat, other wise, whats the point? It would be much stronger with a complete filler that can be nailed though out.
It just seems kinda pointless to make a large hole in the filler, and a complete waste of time, except to remove the dome that was put in the horn to begin with. Unless of course, I'm missing the whole point. I do however, like the idea of putting the wings on the bottom piece instead of the top one....