Lay out the front and back panels for the toy chest on one end of the plywood. Use a tape measure, carpenter's square and straight edge and mark off a 34-by-15-inch section for the front panel. Cut the panel with a circular saw. Repeat the process and cut the back panel using the same measurements.
Mark off a 34-by-16 1/2-inch bottom panel and two 15-by-15-inch sections for the side panels. Cut the pieces to size.
Lay out a 2-by-34-inch section for the top brace and cut it to size. Mark off a 34-by-15-inch top panel and cut out this piece. This allows for a slight overhang at the front of the chest that makes it easier to open the lid.
Measure and cut two 1-by-13-inch support strips to place underneath the top panel. These strips help support the top lid when the toy chest is used as a seat.
Sand the edges of all the pieces with a random orbital sander. Wipe the sections with a damp rag to remove any dust.
Stand the front and back panels on edge and parallel to each other on a flat surface. Ask a helper to hold them. Run a bead of carpenter's glue along the front and back edge of the right side panel. Sandwich it between the front and back panels.
Align the side panel so it is flush with the edge of the front and back panels. Clamp the three panels together with a bar clamp and nail them together with 4d finishing nails spaced 3 inches apart. Sink the nail heads below the surface and wipe any excess glue with a damp rag. Repeat the process with the left side panel to form a four-sided box.
Run a bead of glue along the top edges of the box. Position the bottom panel flat on top of the box and attach it with finishing nails. Space the nails 3 inches apart, sink the nail heads, and use a damp rag to wipe away any glue that may have oozed out. Flip the box over onto the bottom panel.
Place the 2-by-34-inch top brace piece on the back edge of the box. Adjust it so the edges are flush with the back and side panels. Pre-drill eight evenly spaced holes along the back and one hole at each end. Attach the brace to the box with 1 5/8-inch wood screws.
Lay the 34-by-15-inch top panel on a flat surface. Position the two 1-by-13-inch support strips 2 inches in from each side and centered from front to back of the panel. Pre-drill three evenly spaced holes and attach the strips to the panel with 1-inch wood screws.
Set the top panel on the top of the box with the support strips facing the interior of the box. Butt the back end against the brace piece and align the edges so they are flush with the sides of the box.
Position and attach the lid hinges 6 inches from the sides of the box. Lift the lid and attach the interior lid support bracket according the manufacturer's instructions.
Remove the bar clamps. Fill all the nail holes with wood putty and sand the areas. Wipe away the dust with a damp cloth. Apply stain and polyurethane, or paint the box to finish the project.