I made this for my Kids. We have this popular constructor here with plastic bolts and nuts, but as always - there are not enough nuts.

I made also a modified version of the nut, the stl-model is cuttet with an helix-cut. If you slice the model with 0.2mm per slice, you get some kind of a stability structure, the inside hole is connected with the outside perimeter, so you can save a lot of time and filament to fill the whole nut.