This update is a small but vital step in this project. The digi cart is now running under its own power. I have swapped out the bad DC to DC converter. 

This new one can go up to 5 amps, so plenty of headroom. I have also found a micro switch small enough to fit in the case that can also take 1 amp at 5 volts ( max for this circuit ) 

I've installed everything into the Digi Cart to see how it fits and to find out any issues

Issues so far 

1 . Raspberry Pi standoffs need lowering to allow some more space for ribbon cable, as it is hitting the camera module when I try to close the lid fully

2. The SD card reader is the type where the micro SD card sticks out, something I had not realised until I tried it in the case. I will need to rework the case design to move this SD card reader back. 

Before I address these issues, I'm sorting out the camera itself, trying out different ways to mount it, different-sized extension tubes, and making sure I don't need to change the mounting points on the case.