BeanCase

The LightBlue Bean by Punch Through is a small Bluetooth/Arduino board that can do tons of cool stuff.
This is a case that lets you protect it, but is easy to open to get to the battery.
Printed great on an Ultimaker2 using standard quality but the slider track may need to be cleaned in post-process on other printers.

EDIT: Added a lid variant with screw holes. You can attach the lid to the wall ( make sure the screws countersink or the case may not slide over it! ) -- or edit the scad file to move or resize the holes as needed. SORRY the SCAD files are literal; I haven't learned how to use variables in OPENscad yet.