Set two 24-inch beams and two 27-inch beams in a rectangle with the 27-inch beams inside the 24-inch beams. Glue in place with a thin coat of wood glue. Reinforce with two 3-inch wood screws, driven through the face of the 24-inch beam and into the end of the 27-inch beam.
Repeat step one to build a second end section.
Attach the two end sections with one of the 51-inch beams at each corner, the ends of the 51-inch beams abutting the faces of the lumber in the end sections. Glue in place, then reinforce with one 3-inch wood screw at each corner.
Set the frame up so that the 30-inch tall faces form the sides of the frame, with the 24-inch faces forming the top and bottom.
Reinforce the 54-inch sides of the frame by inserting one 27-inch beam on center in each. Glue in place and reinforce with one wood screw per connection, just as you did with the 51-inch beams.
Set one 30-inch by 24-inch lumber sheet on one end of the box so that its edges are flush with the edges of the frame. Glue in place. Reinforce the glue with one 1 1/2 inch wood screw at each corner and one 1 1/2 inch wood screw on center for each edge.
Repeat step one to mount the other 30-by-24 lumber sheet to the opposite edge.
Set one 30-inch by 54-inch lumber sheet along one long side of the box, its edges flush with the frame. Glue in place. Reinforce with one 1 1/2 inch wood screw at each corner, one wood screw on center for each 30-inch edge and six evenly spaced wood screws along each 54-inch edge.
Repeat step three to mount the other 30-by-54 sheet to the opposite side.
Roll the frame over to expose the bottom face of the bench. Set one 24-inch by 54-inch lumber sheet on the bottom with its edges flush with those of the frame. Glue in place and reinforce as you did the lumber sheets in steps three and four.
Roll the frame back to upright.
Set the remaining lumber sheet on top of the frame. Attach it to the frame using the four hinges and accompanying screws. Set the center of the hinges 4 inches, 16 inches, 38 inches and 50 inches from one end of the bench.
Sand all exposed faces of the bench.
Paint or stain the bench to match or complement your existing bedroom furniture. Allow to dry.