1. Build the 8- by 18-inch base from a 16-inch-long piece of 1-by-12, ripped to 6 inches wide, then edged with 1-by-2s.
2. Draw and cut one support piece from a 12-inch piece of 2-by-8.
3. Construct the gable roof so each side is the size of one half of the mud flat. 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 roof sides. The bottom pieces meet at the center peak with edges cut at 60°. The same angle repeats in the mitered side pieces, which should extend 1/8 inch past the bottom edges to create the drainage slots.
4. Center and attach the support on the base. Center and attach the roof. Position the roof so the drain slots clear the base.