I did finally figure out a feeder for my rhea, if anyone's interested. I bought a 15" wallpaper soaking tray and built a wooden frame so it hangs in between the support posts of his shelter. I'm using 2 trays stacked inside one another because the material they are made of is kind of thin.
Just installed it this afternoon. He is so wary of new things it's comical! It holds about a gallon of feed, about 3 times more than the little box he had before.