1. Build the 14-inch-square base from a 12-inch-long piece of 1-by-12 edged with 1-by-2s.
2. Draw and cut two support pieces from 9-inch pieces of 2-by-8.
3. Build the 18-inch-square roof. You'll need two 16-inch-long pieces of 1-by-12, ripped to 8 inches wide, for the roof bottom, and 1-by-3s for the sides. Leave a 1/8-inch gap for the drainage slot at the lower edges.
4. Center and attach supports along opposite sides of base. Position the roof so the drain slot clears the base.