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 find a solution to 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 came up with a dowel nut design that works. It needs to be made of flexible filament (TPU) and pressed firmly into a hole; removing it from the beam hole is therefore 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).