Back part for the original design.

No special instructions, use those provided in the original design or your own. 

I printed the case and back with a 0.3mm nozzle using PLA without problems.

The original back is poorly designed.
The studs that hold the screws break off at nothing. Which also happens as they are too far away from the side of the case.
The back also deforms because of lacking horizontal support.
The cable hole is way too big and not ideally placed as my connectors are not in the center of the board.

So I made a new design and added it as a remix.
The small cutout in the support is because that's where the buzzer on the board is located and it needs a bit more space.
I also add two screw holes more on the horizontal part in case you need more rigidity, they are centered and 4mm away from the edge as the verticals ones are.

Since the case itself is good enough I didn't redesign it.