Make sure again that the screws will not enter the wood farther than 8-9 mm to avoid penetrating into the wood that will remain in the finished plate.
The holder must hold the plate firmly but avoid over-tightening.
Take a look at the red arrows in Fig. 3 to see the general direction of carving.
As you continue removing wood be extremely careful in the area of corners, as these are easily split off. Be extra sensitive of the grain in those areas and apply only very limited force. See the red arrows near the corners in Fig. 3 for the direction of carving. Also make sure your gouge is razor sharp.