Every violin (cello, etc.) should come with a tuning wrench.
Tuning peg sizes vary, so you will need to choose one or the other of these, or scale one to fit your pegs. Consider lining the inside with felt to protect the pegs, as the edges take all the torque.
Violin_Tuner_1.stl needs support for the handle. Simplify3D slicer didn't cut the mustard in this case as you can see in the photo. The finish is ugly, but it's quite functional.
In Violin_Tuner_2.stl, there is built-in support separated from the part by an array of dots. Of course you'll need to add support for the support in your slicer since it doesn't reach the build plate.
Carefully peel the support away. This may take a while with a hobby knife, but is well worth the effort to have a good finished product. Always cut AWAY from yourself with a knife.