The first step before fixing the parameters of your Atomic Pi enclosure is to find a way to power it. The mini board by Digital Loggers is a pain to fit into a box, so I made my own board from a RadioShack(?) prototype board and some spare headers.
The +5V and GND headers to the right are to power the cooling fan. I'm using a 50mm 12V DC fan from my junk box. It seems to run fine on +5V. The blue headers are power in: +12V, GND, GND, +5V.
(I'm not using a +12V supply as it's easier to just plug a USB sound card into an external USB hub.)
I only wired up the most easily accessible GPIO pins (from bottom right, GPIO2/0/5/3), and skipped the serial pins as the board was getting too messy. (If I ever need more than 4 GPIOs I'll add some wires.)
Any prototype board will do, as long as it has 13 rows of holes (if you want to breakout the full 26 pin header.)