This is an idea I've considered doing for a crayfish breeding tank just so I can get more ground area for them. My idea was getting a piece of acrylic and using aquarium sealant to create an entire "floor" halfway up the tank, with an opening of a ramp that extended down to the actual bottom of the tank. I'd also use the sealant and more acrylic sheets to make a wall on the lip of the fake floor to hold water. Then all I'd have to do is pump air under the fake floor. Excess air just bubbles under the wall and up and out of the tank, providing some nice aeration as well. Never had a chance to try it though.
As for making a stand alone container, it should definately work fine. Just need to make sure it's weighed down enough to keep it secure.