Brush waterproof glue on the edge of one board as you construct the two-board L-shaped top of the mailbox frame. Butt the second piece against the glue, and attach it permanently with nails and a pneumatic nail gun. Repeat this step to form an identical structure for the bottom of the mailbox frame.
Attach the frame sides to the top and bottom you have just constructed. The sides are rectilinear boards cut to dimension, but with the forward corners cut off at a 45-degree angle. The back corner remains uncut. Glue and nail the joints.
Use an orbital sander to smooth the surfaces of the frame assembly that is now complete.
