Summary

A very simple case (open) holding a Banana Pi board and a standard 2.5" HDD. Compatibility with original Banana Pi and Banana Pro has been confirmed.
See instructions for a list of parts to print and needed screws.

In the pictures you see my print where side parts are made with green glow-in-the-dark PLA. Looks nice at night.

Instructions

This case is very simple and easy to 3D print. You'll need to print the following parts:
How often you'll need which part

2 times bananapi_hdd_part.stl
3 times bananapi_pi_holder_1.stl
1 time bananapi_pi_holder_2.stl

All parts are designed to be printed easily without support material. The STL files already contain the parts in the best orientation for printing.
Additional required parts

4 HDD screws
4 M3 x 16mm screws
4 M3 nuts
4 2.2 x 6.5 mm screws
Instructions

Fix the 2 HDD parts to the sides of your HDD (with standard HDD screws).
Add the 3 pi_holder_1 and 1 pi_holder_2 parts to the HDD parts and fix them with M3 screws. The pi_holder_2 part is a bit bigger than the other 3, because the holes on the Banana Pi board are not symmetrical. Look at the board to find the one hole that is different and mount the pi_holder_2 part at the corresponding edge.
Last step: put the Banana Pi board on top and fix it with little 2.2x6.5mm screws. If they don't go in easily, try to screw them without Banana Pi board before and add the board later.
Optional: I added felt wheels to the four legs (same stuff which can be used for furniture). This way the whole thing stands softly and possible HDD vibrations don't create noise.
Summary
That's it.
This case is useful for situations where you want to use your Banana Pi with HDD but don't have a place where to put them. And it's not nice to place them somewhere on the ground.

Simple case for Banana Pi + 2.5" HDD