If you haul cargo in an open-air trailer, you may risk damaging the cargo by exposing it to the elements. A homemade plywood enclosure mounted to your trailer may help to protect delicate cargo from weather, and conceal it from view. Cargo trailers tend to have metal stake mounts welded to the inner sides of the frame. You can mount your plywood enclosure to these stake mounts to secure it to the trailer. Using hasp hinges, you can quickly access the rear of the enclosure, while keeping it secured during travel.
Locate the stake mount pairs on the front, rear and sides of the trailer frame. Measure and note the distance between inner edges of the stake mount pairs on each side of the frame. The side distances, and front and rear distances should match.
Measure the insertion depth of the stake mounts. This depth should measure between 3 and 5 inches.
Cut six 1-by-4 boards to a length that matches the distance between stake mount pairs on the sides of the trailer frame. These will be the horizontal boards for the side walls. Cut six 1-by-4 boards to a length that matches the distance between stake mount pairs at the front and rear of the frame. These will be the horizontal boards for the front and rear walls.
Cut eight 1-by-4 boards to a length of 4 feet plus the insertion depth of the stake mounts. These boards will be mounted vertically to the plywood walls, and used to mount the walls to the frame.
Frame the two 48-inch square plywood side walls. Lay three horizontal 1-by-4s, cut for the side walls, at the top, middle and bottom centers of each plywood board. There should be an equal amount of plywood on either side of each horizontal board. Fasten the boards to the plywood with a screw gun, using two wood screws per end.
Place two vertical 1-by-4s against either side of the three horizontal boards on each plywood side wall. Allow the extra length of the vertical boards to extend past the bottom of the walls. Fasten the boards with a screw gun, using two screws each, at the top, middle and bottom of each 1-by-4 board. The 1-by-4 framework should resemble a step ladder with extension legs at the bottom of each plywood wall.
Orient the two 38 1/2-inch by 48-inch plywood boards with the shorter sides running horizontally. Frame these boards for the front and rear walls, using the same procedure that was used for the side walls. Fasten three horizontal boards at the top, middle and bottom centers of each plywood board, using wood screws and a screw gun. Fasten two vertical boards flush to each side of the horizontal boards with wood screws and a screw gun. Make sure the vertical boards extend off the bottom of the plywood.
Align the 40- by 48-inch plywood board flat on the trailer frame. Fasten the board to the frame by drilling 3/8-inch holes that correspond to the frame's screw holes. Attach lag screws using a socket wrench.
Mount the front and side walls to the trailer by fitting the vertical leg extensions into the trailer's stake mounts.
Apply a line of glue along both vertical edges of the front plywood wall. Sandwich the front wall between the two side walls, creating the front corners. Drill 3/8-inch screw holes to accommodate two equally spaced steel angles along each front corner. Fasten the steel angles to the corners using lag screws and a socket wrench.
Apply a line of glue along the top edges of the three plywood walls. Place the 40- by 54-inch plywood ceiling board atop the three-sided box. Allow the 54-inch board length to extend over the rear of the box. Fasten the ceiling board to the front and side boards using two steel angles, equally spaced, along each of the three corners. Drill 3/8-inch screw holes to accommodate the steel angles, and fasten them with lag screws and a socket wrench.
Attach the rear wall to the rear stake mounts on the trailer. Attach two equally spaced rotating hasp hinges along either vertical corner where the rear wall meets the side walls. Fasten the hinge components to the edges of the side walls, and the corresponding rotating cylinder components to either side of the rear wall. Drill 3/8-inch screw holes to accommodate the hasps and cylinders, and secure them with lag screws and a socket wrench. The hasp hinges should latch over the cylinders, and the cylinders should rotate to lock the hinges in place. This setup will secure the rear wall during travel, and allow quick removal during loading.
Apply silicone sealant along the front and top corners, and around all the lag bolts, to weatherproof the box. The ceiling board rear overhang should prevent water runoff into the rear of the box.
