Dowel nuts are one of the most common fasteners; you are probably very close to a few now if you look at the furniture around you. Unfortunately, the STEMFIE construction standard does not allow for a typical dowel nut to function because the holes in the beams are just slightly larger than the threaded screws.
Nevertheless, my constant wish to solve this problem led me to consider an unconventional approach. If the nut is flexible, one can press it into the hole where it can expand. Since most consumer-level 3D printers can use flexible materials, such as TPU filament, I decided to try it.
After a few creative moments in FreeCAD, I devised a dowel nut design that works. However, it needs to be made of flexible filament (TPU) and pressed firmly into a hole; removing it from the beam hole is a bit tricky.
I placed a longitudinal 2 mm hole that goes through the nut. You can use this hole to house a thin axle, a piece of thread, or a bit of 1,75 mm filament. To remove the nut, you can use a small crochet hook to pull this nut out of the beam.
I will soon publish this nut under the fasteners folder, but if you want to try this prototype, you can download the FreeCAD file from this link (right-click and save as).
This toy is unsuitable for children under three years due to choking hazards! 
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