I finally unmolded the bird bath I'd poured a few weeks ago:
I am pleased with the result. It's got the good shape I was hoping for. My only problem is that the bottom ended up uneven, as if there was a "bubble" between the outer and inner saucer when it was curing. So I've spread a layer of mortar down to even it out a bit. I need the surface as smooth and even as possible so that the mosaic isn't a pain.