Double headed knurled screw

I've upgraded the existing model to a double headed screw with the nut captured inside.
This could be a good way to challenge people to try to get the nut out (but be careful they don't break it)
The catch is how to put the nut inside -
With the slic3r (or any other modeling software) position two instances of the screw with some distance between them.

Once there is enough height to the screw, pause the printing while the extruder is on one of the screws, and put the nut on the other (be careful not to move it from the surface). Resume the printing and then put the remaining nut on the other screw.

Notice that the nuts have to be printed before the screws and I recommend scaling them a bit bigger (so putting them on the screw would be easier).