I'll keep the mushroom posts in mind although I doubt I will get much movement on such a small instrument with how I built it.
So I had the idea to hammer form the resonator cover out of the aluminum. Seemed like a good idea, would have been easier with thinner aluminum. Had a piece of fir plywood that I carved the dome shape out of, used a hand plane and worked up a sweat. That was premium old growth fir ply. It was easy enough figuring out if I needed to take off more from one side or the other. Just make nice little circles, sand it smooth.
Doesn't look much like it but there is a dome there. Next file and sand it smooth. Need to figure out some type of hole pattern to let the sound out.