This is a case for a common OpenAPS implementation by Robert Silvers and Eric Burt.
NOTE: If you have a new v1.3 version of the Explorer board that started shipping in July 2017, please use the file that says 1.3. The new board is thicker and wider.
There are versions for a single, or double Adafruit 2500 batteries. For the dual-battery version, you will need to construct a Y-Adaptor using 2.0mm 2-pin JST connectors (10 pair are about $9 on Amazon https://www.amazon.com/gp/product/B072B66P5C/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1 ). Note that in the photos, the red/gold "Iron Man" version is the 5000 mAh size.
I got 19 hours of battery life with one battery, and 32 with two.
Cooling is very important for the Edison/Explorer, and this has vents. There are several bottom piece choices - with and without a belt clip.
If you don't have a printer, or do but want a pro-quality print, you can order a commercial-quality print of it from my Shapeways store:
The enclosure expects the Explorer and Edison to use 3.0mm spacers.
The AAA battery holder is for a spare Medtronic pump battery.
Print the two halves open-face up with support on.
It is 75.5x52.2x21.7mm.