Is there an easy way to produce a block representing the interior space between 2 parts of an assembly?

I've attached the assembly. This is a fan shell in 2 halves with feet on the bottom, handle and control cavity on top.  I need something, a block, that represents the space available between the shell and the top where we can cram in as much electronic circuitry as possible.

How do I go about getting that?  Is there a tutorial?

jerry watts