Lay two planks of 10-by-17 inch MDF on a 17-inch edge, 35 inches apart on a work surface. These are the legs of the bench, which are currently resting on their sides.
Slot one plank of 10-by-35 inch MDF between one end of the legs, making right-angled corners with the legs. Slot a second piece 6 inches up from the other end of the legs.
Drill four countersunk pilot holes, evenly spaced, from the outside of the legs into the abutting ends of the 35-inch planks. Secure with 1 5/8-inch screws through each pilot hole.
Turn the bench so that it is upright (onto the 10-inch edge of the legs), with the 35-inch plank that is flush with the top of the legs at the top, and the other 35-inch plank, the bench's storage shelf, 6 inches off the ground.
Center a plank of 12-by-40 inch MDF over the top of the bench. There should be an inch over along the sides and 2 1/2 inches over at each end. Secure the bench top to the bench with 1 5/8 inch screws drill from the underside of the bench into the underside of the bench top every 5 or 6 inches around the perimeter of the top 35-inch plank.