Well, this is my first attempt.

This is a capodastre for folk guitar but it might also work for classic.I haven't printed it yet because I haven't finished my printer. No idea if it's thick enough to resist pressure. But if anybody feels like trying...

Okay, first download the "resized" STL, (the other is too small).

You will need a good wooden clothes peg for to get the spring. The one I used for sizing is on the picture. Diameter of spring is 7,8mm about. You should be able to attach it with pliers.I had to redrill the hole where the spring is attached to the bottom part.

Some large rubber band could be glued with 2 components glue along the inside in order to protect the chords.