Measure the inside depth of the cabinet. Measure the height from the shelf to the top of the cabinet. Cut one piece of 1/4-inch oak plywood to this measurement. Cut two 1/2-by-1-inch strips to the same height. Measure the height from the underside of the shelf to the bottom of the cabinet. Cut one piece of 1/4-inch oak plywood to this measurement. Cut two strips to the same height.
Place your cut plywood on the work table. Apply a bead of glue to the 1-inch side of the companion strip of wood. Align the wood to the side edge of the plywood and clamp the pieces of wood together. Repeat for the second side. Repeat for the lower plywood and its sides.
Drill through the strip of wood into the plywood for a pilot hole. Drill three holes in each strip evenly spaced apart. Screw wood screws through each hole to hold the strips to the plywood. Wipe of any excess glue. Repeat for all of the strips. Allow the glue to dry for 24 hours.
Unclamp the pieces. Stand the cut and attached plywood on its strip end. Avoiding the three screws, drill two pilot holes through the opposite side of the wood at least 8 inches apart. Repeat for all strips.
Position the above plywood on the upper shelf with the strips toward you. Align the board with the area of the corner cabinet you wish to block off. Use a level to hold the board straight. Screw through your pilot inside holes into the front and back walls of the cabinet. Repeat for the second below board. This will block off the unreachable areas of the cabinet without disturbing the shelving.