Using a relay to control power:
http://www.thingiverse.com/thing:1726120/#comment-1134637
Bought this relay:
http://www.ebay.co.uk/itm/252614067501
Piggypack Pi's power from UM2 PSU:
https://ultimaker.com/en/community/15515-powering-raspberry-pi-from-ultimaker-2-main-board
So made my own mount:
http://www.thingiverse.com/thing:2117493
Stand for camera:
http://www.thingiverse.com/thing:31171/#files
Finished result, bottom:
In action:
https://goo.gl/photos/s2cpEuw1YVpdgWJR9