Decide on specific measurements for your bench. The width of the bench will depend on the size of the dugout and how much non-bench room you'd like on either end. The height and depth of the bench also needs to be decided. A youth-league bench is commonly sized at 30.75 inches high, 78 inches wide and 12 feet long. A bench for older players is 5 inches higher, 12 inches wider and 6 feet longer. This size, in the end, is up to the builder.
Design the bench to be backed flat against the wall of the dugout. This way, players who sit on the top tier of the bench can lean against the back wall. Also, if items are placed on the top tier, they won't fall behind the bench.
Construct a bench with a flat-seating surface and another flat surface to sit on the top tier of the bench. When building the legs of the bench, it may be a good idea to plan on digging and planting them into the ground, unless you're building for a concrete floor.
Build the bench seat and back so that the parallel boards are very close together. You don't want open gaps on the bench because items like baseballs, batting gloves and water bottles can fall through.
Use a coat of water-repellent finish on the wood so that it is better suited to stand the test of time. The last thing you want after building benches in your dugout is see them warp and crack after a few years' worth of rainouts.