Here's a single-disk NAS box using Banana Pi
The Banana Pi (first edition) has SATA port and gigabit ethernet so it's a good choice to make a NAS.
You can even use a SATA Multiplier to connect multiple SATA disks to it. (But it won't be as fast, as they will share the same SATA bandwidth)
The box is a tin box, sold as a "Diswasher Tablet Box" aimed to store diswasher detergent tablets in.
They will be powered with a "Molex Power Adapter".
Step 1: A Dishwasher Tablet Storage Box With a HDD Mounted On...
The HDD's mounted onto the box using L-brackets. There's a 120mm fan on the bottom and the bottom's roughly cut out using a drill and pliers. Using tin snips is recommended, mine wasn't around at the time.
Power's distributed to the Banana Pi and the fan using plastic screw terminals (blue in the picture)
The ground wires (black) are connected together as well.
Step 2: Support the Pi Enclosure
Took a pair of the plastic things (had them lying around in the toolbox) and connected them to the fan so that there would be some support for the Pi enclosure. Could also use L-brackets, or Sugru, or maybe even hot glue.
I used two washers per screw (one in the bottom and one on top) to better support the weight.
Step 3: Put the Banana Pi Inside
Put the Banana Pi inside and use tape on top for support. I used Kapton tape which is thin, strong and sticky.
Tape down all the loose cables inside, to minimize the risk of them getting caught in the fan.
Snake the power cables between the L-brackets and the HDD for extra support. So that an unintentional tug on the power cable should be caught by the bracket, and should not put stress on the taped-down parts.
Step 4: Final Result
Use a Molex Power Adapter to power it all.