I've decided to separate the Arduino and Raspberry Pi into different cases - as a space saving measure. I don't want this device to become too large for the wrist, to the point where it would become uncomfortable. 

So I will have one case which will be the 'watch' which will house the Raspberry Pi, battery, speaker etc. And another case which will house the Arduino and sensors circuitry. This case will sit on the back of the hand.