On either side and back of the head are these pockets.
These are the pieces for each pocket. The sides go the full height of the head and the small blocks are used to form the top and bottom edge of the pocket. These pieces are rounded slightly on one side to match the curve of the head. I also slightly rounded the edge of the side pieces, again to match the head curve. Finally there is a piece of 1/4” luan that forms the back.
Here are the assembled sides and back. Ignore the yellow, it was just a piece of wood I had laying around that was already painted.
Before installing the finished pockets in the head I painted them with some white spray primer.