I hope this worked. I won't know until it sets up and I can unmold it. Rather than purchase an actual bird bath mold, or use something like a terra cotta saucer (wouldn't that have been easy!), I chose to fashion my own bird bath by using these two saucers:
It was a way bigger pain in the butt than I thought it would be. It will be worth it if I unmold it and find a beautiful bird bath saucer inside.
And note to self: A 60lb bag of mortar makes a CRAPLOAD. I ended up with a bird bath top, 4 stepping stones and a mushroom cap. Oh, and still in the workshop is the second bag of mortar I'd bought thinking I might run out just making the bird bath top. I don't know what I was thinking. Even if I'd needed the second bag, what the hell would I have done with a 120lb bird bath??