Purchase good quality, rot-resistant wood from your local hardware store. You can select cedar, oak, pine, redwood or teak, but the final choice should depend on the allocated budget. However, keep in mind that good quality wood lasts a long time, and your planter box will serve you through many cycles of crops.
Use a circular saw to cut two 36-inch long, 20-inch wide pieces for the front and the back of the planter box, and two 24-inch long and equally wide pieces for the sides. The fifth piece--the base piece--should be as wide as the short sides, but equal in length to the front and back pieces.
The root system of some berries like blueberries get entangled as it spreads, so a box this size will provide it enough space to grow and spread.
Drill 1/4-inch drainage holes on the base piece of the planter, spaced evenly apart. Rub medium grit sandpaper over the holes to remove any sharp edges or splinters.
Take a side piece and hold it flush against the edge of the front piece. Make sure their tops are at the same level, and join them together by drilling two 1-inch galvanized wood screws into the long piece that will penetrate the short edge.
Repeat the process to join the other side piece to the edge of the back piece. You will now have two L-shaped structures.
Dry fit both the structures in such a way that the short side of one is flush against the long side of another. Make sure both the joints are flush and even. Use your drilling machine to drill two 1-inch galvanized wood screws to join both structures together.
Review the box and make sure it does not wobble, otherwise reinforce any weak joints by drilling another woodscrew to secure it.
Turn the box over so its base cavity faces upwards. Lower the base piece with drainage holes over it, and fit it into place by drilling two screws in each short side, and three in each long side.
Turn it over and check to make sure the base is secure and will not come loose with the weight of the soil. Otherwise, reinforce it by drilling another wood screw in each side.
Brush the surface of the planter box with sandpaper to remove any sharp edges, giving it a smooth finish. Do not forget the inner corners or outer joints. You can personalize your planter box by applying a coat of exterior paint or clear acrylic paint. Allow it to dry thoroughly before filling it with good quality potting soil.