Measure the height of the space under your bed. A basic bed frame typically provides six to eight inches of space.
Cut two 16-inch lengths from a 1-by-4-inch board, using a circular or hand saw. Cut two 24-inch lengths from the same size board. You can use a wider board if your bed has a taller space beneath it.
Stand the boards on their narrow side to make a rectangular box. Set the boards so the longer two boards are between the shorter boards. Screw or nail the box together at the corners.
Cut a 16-by-26-inch rectangle from a sheet of 1/2-inch thick plywood, using your saw.
Lay the plywood piece on top the organizer frame, lining up the edges. Screw the plywood onto the frame to make the bottom of the organizer.
Screw a 1- to 1 1/2-inch tall caster in each corner of the plywood drawer base. Predrill the holes prior to screwing in the casters to prevent the wood from cracking.
Drill a ¼-inch hole in the center of the long side of the organizer. Attach a drawer pull through the hole. Use the drawer pull to pull the organizer out from under the bed when you need to access it.