I bought some 2x4's and cut them to size. Measuring correctly is so key! Remember, 2x4's are actually 1 1/2" x 3 1/2". I then used 2 1/2" screws and wood glue to secure them. Because they need to go through 3 1/2" of wood you have to predrill and counter-sink the screws. It's a pain, but the only way to do it.
The above picture is for the bottom of a bench.