I designed this for keeping my classical guitar quieter while practicing around others. It lightly mutes the strings; almost sounds like a palm mute. It will need to be scaled in your slicer to fit your guitar properly. I made this one about the same height as from the body of the guitar to the top of the strings, so there's about 1mm of "squish" on each string. (My strings are all pretty close to the same height over the body)
I printed this in ninjaflex, but other flexible filaments will probably work fine. You might need to experiment with height to get the desired amount of muting. I printed with 2 walls, 2 bottom layers, 4 top layers with a 0.4mm nozzle.