If you go with the two rubbermaid setup, you may want to fill the gap between the bottom of the top one, and the inside bottom of the first one. I don't know how well the centre of the top one will do under the pressure of the water, (but then again, you could get lucky, and it will just bow down a little and no real problems... and you do have a second one to catch any leaks from the first
).
Robert