Solid Baltic Birch Plywood Inserts. Set of Ten.
6.5" (16.5 cm) inserts for our 12" Lucky Bat holder are made from 6mm (1/4") thick plywood.

Wood fiber swells when it gets damp. After using and drying the inserts for the first time, you can remove the raised grain with fine sandpaper. Subsequent use will not raise it again and the surface will develop a patina which is smoother than new.