Meniscus mirror mold attempt #5: success!
Bought some new furnace cement. Reused the same mold as before. Gave it four coats of spray polyurethane, sanded with #320 sandpaper before the last coat, taped up paper to the edge, mixed and poured cement and hurt my hand when my 3D printed spatula cracked in half. Lots of water ended up leaking out of the sides, which I was worried would be a bad thing and dry it out, so I added more water on top after the first day.
But it looks like it worked! Getting it out of the wood mold was a nightmare, I went around the side with a blade and it didnt help. Then I wised up and brought it outside, turned it upside down, gave it some thumps and flexed the wood, and it fell out.
It looks so smooth! Hopefully now I can move on to actually slumping glass!