I'd use half inch plywood and 2x4s. Build a frame with the 2x4s about 4 feet above the top of the bed of the truck, attach plywood with 2 1/2 inch decking screws to the frame on the front, and sides, then take the tailgate off, and use another piece of plywood with 2 16x4 vents covered in mesh and a hole for the discharge to shoot into the box, and attach that to the frame with hinges so you can either swing it out of the way when you are ready to unload, or take it off completely. If you are not planning on getting on the highway and/or interstate you could take the box up another 2 to 4 feet. To give yourself quite a bit more room. That is about the simplest box you could ask for
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And I'm 18...