As Jim stated, the 40 gallon rubbermaid tank will be large enough. You can house baby tegus in a 30 gallon aquarium for a few months before they really need a larger enclosure. I have some care sheets on Tegus, located at theTegu.com
The only thing I would worry about is how to seal the container. You could use a screen, but a screen isn't normally very durable and a determined cat or a dog can get through a screen. The hide, mulch, etc is required as stated but so is proper temps and a basking light. If you use a screen you cant sit a basking light on top of it so you would probably have to hang the light with a rope or get a clamping type.
Just a few things you need to keep in mind.
Again please feel free to visit and join theTegu.com, a new Tegu community.
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Rick
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