Things You'll Need
Instructions
Measure the 1-by-4 boards and cut them into four pieces with the miter saw. Two should measure 16-inches long, the other two, 20-inches long. Make sure each piece is cut at a 45 degree angle so they will fit at a perfect 90 degrees to make a securely fitting frame.
Cut a groove in each plank 3/4 inches from the bottom along the entire 20 or 16-inch length. Make the groove just wide enough for the wire mesh to fit securely inside it.
Dry fit the boards together to check that the mitered edges make a sharp 90 degree corner. This will give you a rectangular frame. Drill pilot holes in each corner. Disassemble the frame and cover the mitered edges with waterproof glue and fit them together. Swap out the drill bit for a screwdriver bit and drive screws into the pilot holes.
Cut the wire mesh to 15.25 inches by 19.25 inches. Slip it into place in the grooves you cut. You may have to bend it a bit to get it to fit. The pressure of the wire into the frame will be what keeps it in place, but you may secure it with a screw through the top of the frame if you wish. Just be sure to make pilot holes with your drill bit to prevent the wood from splitting.
Treat the frame with waterproofing wood treatment.