The shelves are 21" x 16" so they're perfect for my components.
So your inside cabinet width is roughly 21.375 inches. How much width do you require? You can either laminate .05" or .75" plywood to each inside cabinet wall, allowing for a precise dado for each shelf. Using .05 plywood will leave you a rough width of 20.075'; using .075 plywood will leave you roughly 19.875".
You can purchase birch plywood (which nicely accepts stain and can be matched very closely to maple), the width of which should be ripped to the same spec as the inside depth of the cabinet walls -the length of which should be cut to approximately the same spec as the inside height of the cabinet. Now, figure shelving positions and mark the inside of the cabinet (precisely). Then cut your pre-ripped plywood to the necessary lengths to fit above and below each shelve (leaving some room for the glue/cement). Use quality wood glue and screws to secure the plywood to the inside cabinet walls. Drill/countersink screw holes in the plywood so that you secure the screws from the inside of the cabinet (which can be covered with dowels best, or with wood putty wont look professional). Insert your shelves by sliding them in place via the rear of the open cabinet. I recommend using an acoustically designed speaker cement or silicone to glue the shelves in place.