Our jello mold is a cylinder with an acrylic bottom hot-glued on one end. The problem is, we weren't confident it would be water-tight if the entire tube was filled with liquid jello. So, we filled it to two inches deep and let that set, first, to plug the end. You don't see it in the photo, but we let the entire mold sit in a tub filled with water, to equalize the pressure against the jello as it set.
That's Doug's cat Mackerel.
(Click the image to view a larger version.)