On a whim, I stopped into the Lego store recently and I bought this Minifigure Ice Cube Tray for $7. You can’t get them online (submit your email address for when they do become available if you don’t have a Lego store near you)
Here’s the tray from our house, you can see two of the figures in ice. My boys are loving it! $7 in-store at Lego.com. They also make a Lego Brick Ice Cube Tray too.