Dig two 12-inch-diameter, 24-inch-deep holes so their centers are spaced 107 inches apart. Place a 78-inch board inside each hole and pound two stakes around each board so they are held vertically. Use your level to ensure they are vertical. The two boards must be aligned.
Pour cement in the holes, around the boards and let it cure according to instructions.
Lay two 115-inch boards on edge, parallel and 3 ½ inches apart. Position a 22-inch board on edge and against each pair of the ends of the 115-inch boards. The ends of the 22-inch boards should each be 7 ¾ inches away from the sides of the 115-inch boards. Screw four screws through each 22-inch boards so two screws enter each end of the 115-inch boards. This is the top frame to your mailbox holder.
Lift the frame up and over the 78-inch boards. The 78-inch boards should be between the 115-inch boards and the 22-inch boards. The top ends of the 78-inch boards should be flush with the edges of the 104-inch boards. Screw through the 22-inch boards and into the 78-inch boards. Use four screws for each 22-inch board.
Set the last two 115-inch boards perpendicularly on top of the 22-inch boards so their ends are flush with the sides of the 22-inch boards. Space the 115-inch boards 4 inches apart so their sides are flush with the ends of the 22-inch boards. Screw the 115-inch boards to the 22-inch boards using four screws for each board.