assign a new lambert to the cube and then go to it's attributes and change it's opacity type to "Black Hole" then render them both on one layer.
The other more flexible option is to render the cube as a separate layer as you have done and then use it as a matte in after effects.
It's up to you really, if you're less experienced then go for the first option.
Alan
Technical Director - Framestore
Currently working on: Your Highness
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