Use a screwdriver to remove the screws securing the Masonite.
Remove the Masonite.
Put the piece of Masonite you just removed on top of the new piece of Masonite.
Use a pencil to draw the shape of the old Masonite onto the new one.
Use a jigsaw to cut the new Masonite.
Put the Masonite on the work surface.
Use a counter sink drill bit to drill a hole in the Masonite and create a 1/8 inch deep counter sink.
Screw the Masonite in place.