Over the last week or so I've been working on how I'm going to power this project. The two options that I could have chosen are standard alkaline batteries or a LiPo and charger and for the sake of simplicity and cost I've decided to go for alkaline batteries. My tests have shown that three AAA cells should give a few hours of run time so I've been working on a battery holder. Here it is:

The positive terminals are made out of the end of a stackable spade connector:

Both types of terminal slot into recesses in the base of the printed part after being soldered together. The outer case stops the terminals from moving outwards. This is roughly what it will look line in the case:

I've had to make the case slightly wider to accommodate the holder but I'm still within my credit card sized footprint with 1mm to spare on the width.