Buy two wooden planters that are approximately 20 inches high and at least 12 inches wide.
Measure the width of the planters. If the planters are angled inward at the bottom, take the width measurement from 3 inches below the rim of the planter.
Cut two pieces of 2-by-2 lumber to the length of the width measurement you made. These lumber pieces are the cleats for supporting the seat between the planters.
Clamp one cleat to one side of each planter, 3 inches below the rim. Use a level to ensure that the cleats are flat.
Drill four pilot holes through each cleat and through the planters. The pilot holes should be sized for bolts that are long enough to go through the cleat and the planter sides. Secure the cleats to the planters with bolts. Tighten the nuts of the bolts with a wrench on the inside of the planter.
Measure how many slats of 2-by-3 lumber can fit between the cleats, allowing a 1/4-inch gap between slats. Cut as many slats as can fit on the cleat to 40 inches long.
Position the slats between the two planters, with the 3-inch ends sitting on the cleats and flush against the planter sides. Secure each slat with a 3 1/2-inch deck screw through each end of the slat onto the cleats.