Place two pieces of particleboard side by side on a flat surface. Arrange four metal brackets down the length of the pieces, evenly spacing each bracket. Place the brackets so that the metal rests evenly across the boards. Screw 1/2-inch wood screws through the holes in each bracket. The two pieces serve as the base for your mattress.
Create a box shape by arranging two 77-inch-long boards with two 68-inch-long boards. Hold the corner where two of the boards meet and screw a 1 1/2-inch wood screw through both pieces. Use four screws spaced along the boards for maximum stability. Work your way around the box, attaching four screws in each corner. Cover the screw heads with a tiny amount of wood filler.
Cover one side of the frame with a thin layer of wood glue. Unroll the quilt batting and press against the wood. Trim off the excess batting with a utility knife. Wrap the quilt batting and wood with upholstery fabric. Attach the fabric to the wood with a staple gun. Repeat the process on each side of the frame.
Set the particleboard piece down so that the metal bracket side faces upward. Place the wood frame you just made on top. Attach a metal L-bracket to each side of the bed frame, securing the box frame to the particleboard top. Screw wood screws through the brackets and into the wood.
Turn the bed over so that the flat side of the particleboard faces upward. Attach the four remaining planks to the top of the particleboard. Arrange the pieces so they fit together on the surface. Screw wood screws through the wood, spacing the screws every 2 to 3 inches and covering with wood filler.