As you've probably discovered, there is a "roomy" area right at the back behind that module under the rear of the rack. However, to access it necessitates the removal of the whole rack every time, so only really any use for things you don't need access to, such as a fuzeblock or other wiring related gubbins. Also, you will have to be careful with what you place there as anything that might cause electromagnetic interference to that module might also cause problems.
As you haven't said why you need to make the room, it's dificult to know what to suggest exactly.
I can't remember who or in which area of this forum it is but someone posted a thread about something similar and ended up hollowing out the pillion seat or rider seat (can't remember).
I must confess that I thought the job was a butchery, looked awful (even if not visible once on) and didn't convince me. Also, he kept tools in the hollow which sometimes become magnetised and I'm quite sure not at all good for that all important module (or even the key module, for that matter)