Z axis on ultimaker is kept only by the bottom coupler and could, in few cases, show some lateral movement that is compensed by the movemment in the XY plane of the Z brass nut.
Just to be on the safe side, this little object can help to keep the upper end of the Z threaded axis in place removing any residual off-axis movement.
int the 2nd picture, on the left the printing of a test box 25mm high, WITH the anti_z_wobble, on the right the original print...
print as you wish, (mine is printed at 0.3 layer, seems coarse but the coarseness help to reduce friction), put over the z axis hole and keep in axis using a steel 12mm rod. sign the place for screw holes and then drill 2 small 2mm holes.
to mount it you need to remove the 2 steel rods and open the coupler. put some grease on the ANTI WOBBLER and put it on top of the threaded rod. carefully, put it in place, and fix it ONLY when it is in axis with the coupler, with the aid form a steel rod.
seems complicated, it is not.
mount againt all the Z stage and start some Z jogging just to diffuse the grease and make the threaded end build a sit int the PLA.
it works quite good.