Cut four sheets of 3/4-inch-thick plywood 36 inches long by 36 inches wide, using a circular saw. Cut two sheets of 3/4-inch-thick plywood 36 inches long by 34 1/2 inches wide. Stand one 36-by-36-inch sheet vertically on the edge of another 36-by-36-inch-long sheet so the edges are flush with one another. Screw the sheets together using three equidistant L-brackets.
Screw a second 36-by-36-inch sheet on the opposite edge of the bottom sheet with three more L-brackets. Screw one of the 36-by-34 1/2-inch sheets between the two vertical sheets, flush with the one edge of the bottom sheet, using one L-bracket per edge. Screw the other 36-by-34 1/2-inch sheet to the opposite edge of the bottom sheet with three more L-brackets, to form the cabinet.
Cut four strips of draught excluder tape 36 inches long. Cut the ends of the strips at 45-degree angles. Remove the backing strip from the first strip of draught excluder. Stick the strip along the top of one of the sides of the cabinet. Stick the other strips to the top of the other sides of the cabinet, so the cut ends are flush with one another.
Cut six sheets of soundproofing felt 34 1/2 inches long by 34 1/2 inches wide. Apply PVA glue to the back of four felt sheets. Stick the first sheet to the bottom of the inside of the cabinet. Stick one sheet to the inside of three sides of the cabinet.
Screw two hinges to one of the edges of the remaining 36-by-36-inch sheet. Apply PVA glue to the back of one remaining soundproofing felt sheet. Stick the felt sheet to the middle of the remaining 36-by-36-inch plywood sheet on the same face that the hinges are screwed to.
Screw the hinges to the inside face of the unfelted side of the cabinet. Apply PVA glue to the back of the remaining soundproofing felt sheet. Stick the sheet to the inside of the remaining unfelted side of the cabinet. Screw the cabinet door handle to the outside of the cabinet lid, on the opposite edge to the hinges.