I've made this case when i heard about those cheap chinese bluetooth boards. I had a spare drone lipo lying around which i used for this project. 

Everything in there is just pushed into and not glued so you could swap everything right out without having too much effort put into it. 
Its designed to be screwed together with M3x10 screws, the head of the screws will be completely tucked away in the base of the case and have a very sleek and minimalistic bluetooth receiver.

To be on the safe side of things ive also thrown a charging IC into the unit since i didnt trust the bluetooth board charging the lipo properly.

Feedback greatly appreciated