Ready to use STL and parametric source files are included there. Longer description is in the thingiverse, but simplified this:
simple/minimalistic, it would be tricky to make it smaller (no bulges, no fancy curves, or other things making it bigger to carry around)
as a side effect of the simplicity it should be faster to print compared to a more complex design
has ventilation in the mind (the board will not suffocate even in long run)
the fan is protected from fingers
the top is supported by pillars resting againts the PCB (and can be used to screw the top againts the PCB, small self-tapping screws are ideal as some of the holes are under-dimensioned on purpose as it’s easier to remove material)
everything important is accessible by hand (you can see all LEDs, flick switches, plug/unplug connectors by hand and the case shouldn’t be in your way and there is plenty margin for bulky connectors as well)
No hassle, but actually I was struggling with the license, I wanted something which will allow everybody do whatever they want.
Would be interesting to make mod to this case to hold against your fanless heatsink. Could be smaller case and I think in the end it would be better long term design, as I think that tiny fan will die eventually. But probably you need to CNC mill few fins into the material, so the surface area will be much bigger, not just a mass to soak the heat in.
The parametric source file is included but it made with SolidWorks. I always liked ideas of OpenCAD, POV-ray and other programs where you describe the model (or can use script to generate it), but was never good at it and always for more complex models had to revert to something I see and I know how to operate.