It is time again, and I would like to give a small update of the design iteration process. In the meantime, I have completely redesigned the model, made some test prints of it, and am currently at iteration 3. This will probably remain the final version, with a few minor adjustments.
Here are all three of them in comparison. The first version was the one already presented above.
I think the design is not bad, but it has many disadvantages. For one thing, there are a lot of small and delicate parts, and for another, everything has to be glued together. Once assembled, you wouldn’t be able to take it apart again. That was a thorn in my side from the beginning.
During my research for original plans of NeXT or detailed picture material, I came across an already realised project by Nina Richards. She achieved this quite elegantly, as the whole thing was a modular kit. I liked it so much that I decided to adapt it.
My NeXTcube is just a bit bigger (1:2) than hers (10x10cm). I also wanted to include more details, and stay as close as possible to the original.
And so iteration 2 was born:
I liked that a lot better! However, there was still one small thing that bothered me. On YouTube I found a video where someone disassembles an original NeXTcube in an explanatory way and with many close-ups. There you can clearly see that the housing actually has three larger parts. But in my version there were only two.
And so iteration 3 was created:
This may all seem a bit big in the pictures now, but as I said, this is a scale of 1:2 - so an edge length of 15.25cm. 
Next, I need to do a bit of fine-tuning. The Raspberry Pi should sit on a riser card, as in the original, which is then pushed into the housing.
UPDATE: I forgot to mention, that iteration 3 doesn’t need any glue! It will be held together with just 6 screws.