Id start with a water tight bottom, and put a cage on top of it. It will be moderately hard to "make" a water tight bottom that will survive leak free for 5-10 years. This could be done with any number of industrial containers such as a rain barrel or even a concrete mixing tub. I made mine (bare in mind a very low spousal approval factor) by cutting the top off of a IBC 275 gallon tote, but it did create a water tight 48x48 base.