Floating via the lift due to the liquid displaced by the steel container, the way boats and ships do. When that is taken to extremes, obviously there are other issues to take into account, as you pointed out. Will it even be possible to make water form as a one molecule thick layer, or will surface tension turn the task similar to trying to push a balloon through a narrow crack.
Using oil instead of water could be an option. It's slightly less dense, and thus has less lift, but has no surface tension so it will spread out to a more narrow layer. With oil, what I get is that you'd need 3.6 cubic millimeters of oil coating a a hemisphere-shaped hole in which the hemisphere-shaped steel thingy floats. The steel would need to be about 0.28 mm thick. Feasible of course.. it's in the same ballpark as gold foil. Whether it would retain the hemisphere shape is another matter.