Finished draft of add-on board.  Also:

main board
* added heat sink to main voltage controller
* shifted LEDs slightly up (for better alignment with add-on board)

add-on board
* positioned LEDs and detector so they'd be aligned with main board
* created 2mm space around detector/LEDs to make it more 'sealable'
* board is same width as main board

WIP:
Probably need to flip the pins on the 12 pin
connector on the add-on board since it's
upside down