Then with a knife, wood chips central location, about 0.5 cm away from the hole location of the beginning, cut out, the wood thinning near the side of the body, the other side to maintain the original thickness, as shown
The same is true back processing, wood whittled to a tilted surface. We look from the side effects:
The other side also did this, then we just hit a small wooden stick fixed to the hole inside. If it feels loose, then glue it to use it!