The Humidiplug can be added to many open sound hole inserts to give then maximum feedback reduction when playing on stage.
It also doubles as a guitar humidifier.

This sealing plug will fit most 4" Lacuna soundhole inserts. 
It is constructed out of black laminate bonded to super resilient urethane foam. The humidifier is made from layers of Viscose fabric and polyacrylimide gel.