I took the spectrum of an EM172 connected to my amplifier inside and outside a large box filled with foam layers:
I also made a diagram with my plan for the microphone amplifier boxes. This is a bottom view:
The dimensions I got from this box: http://www.digikey.com/product-detail/en/bud-industries/CU-4472/377-1476-ND/696705
This seemed like the size I was looking for and it has a mounting flange that could make suspending it easier. Let me know if you have any suggestions.
I'll be doing a Huddle test next week to get a better idea of the noise floor and well as starting construction of the circuits to go inside the boxes and the boxes themselves.