Position the plywood board on top of the four legs, with the legs pushed into the very corners of the board so both outside edges of the legs are flush with the outside edges of the coffee table. Stagger the leg positioning by moving the legs closer toward the center on coffee tables larger than 3 feet so the center does not sag.
Predrill three holes down through the plywood board and into each leg. Attach the plywood to the legs with the 3-inch wood screws.
Set the xylophone on top of the assembled table, and drill holes up through the bottom of the plywood and into the xylophone base if a permanent attachment is desired. Predrill at least one hole in each corner of the xylophone and more if using a larger xylophone. Attach the xylophone to the plywood with 1 1/2-inch wood screws inserted from below.
Place the Plexiglas sheet on top of the xylophone, and mark four locations near the corners where the Plexiglas will be bolted down. Remove the Plexiglas and drill a 1/4-inch hole at each point. Insert a bolt through a washer and into each hole. Set the Plexiglas back on the xylophone, and mark the locations of the four bolts on the plywood. Drill a 1/4-inch hole at each mark, then pass the bolts through the holes and fasten them on the bottom of the table with the wingnuts.