A removable insert for an opening in a wall board of a string instrument, includes a side wall having dimensions less than those of the opening for insertion in the opening, and a flange wall having dimensions greater than those of the opening so as to seat on the wall board. An arrangement is provided for releasably securing the removable insert to the wall board such that the side wall is positioned in the opening in the wall board, the arrangement including two wing brackets that are rotatably mounted to the side wall for movement between a first position within the confines of the insert in which the insert is removable from the opening and a second position in which the wing brackets are secured to an undersurface of the wall board to lock the insert in the opening.