This Soprano size ukulele is a 3 piece design:
It has a groove in the neck to accommodate a neck rod to stiffen it as most 3D printed plastic is too bendy under string tension.
Use something like this for the in-neck rod:
Also it is meant to be fitted with simple friction tuners like this:
You can hear a demo of it on Youtube https://youtu.be/MWWb5VRRV_g