Measure bookshelf space and cut a plywood panel to size with a saw. Remove the back of the hand with a bread knife.
Place mask and hand on the panel and glue in place, as shown in Figure 1.
Mix papier-mache mix following the manufacturer's instructions. With moist hands, add papier-mache to the panel, as shown in Figure 2.
Spread the mixture, smoothing with the spreader. If your mask has open eye sockets, lay a damp piece of paper toweling over the openings and smooth the edges. Allow the papier-mache to dry slightly on the manicure hand's fingers before covering completely.
When all the components are covered, let dry. Save leftover mixture in an airtight bag for a light skim coat, if necessary for a smooth finish.
After all coats are dry, sand any rough edges or areas with sandpaper. Paint the panel to match your bookcase, as shown in Figure 3. Let dry and slide into place.