The compartment is the size of polaroid photos. The two semicircular "cutouts" left and right are space so you can take the polaroids out of their compartment without the need to flip the book upside down.
• The way I approached this was to clamp all the pages that I intended to cut, using two bar clamps and a double layer of balsa wood on each side, as to not leave marks on the book itself. Having the pages clamped together gives you a stable surface to work on. Using a matrix for reference I cut the pages using an xacto knife. The cutout compartment was a few millimeters smaller than the size it needed to be, in order to have material to compensate for any potential mistakes. The last few millimeters were removed using a hobby drill and a sander bit to get a really smooth finish.
• I did not want to glue the pages together one by one, nor I wanted glue to be visible on the outside of the book. Using a brush, PVA glue was applied to cover all the sides of the compartment that was cut and sanded. Some pages were behind to hide the fabric that I would stretch over the cut. When the glue dried, I applied a second layer and stretched the fabric over the sides as seen in the pictures. Satin ribbon was glued to hide the seams of the fabric in the corners of the compartment. The pages that were left behind covered everything up to leave the compartment looking neat and tidy.
To this day I believe I have not one-upped this gift! :)