I used one sheet of three-quarter inch plywood, I anchored all the sides to the bottom, With liquid nails and wood screws. Then used corner brackets to give it even more support when I'm bouncing around offroad.
. I use turnbuckles anchored to the rear seat brackets To hold it in place. They provide opposing forces and work very well. It also provides a very quick removal, if you have wrenches. I used lock nuts to lock the buckles in place, That way they don't vibrate loose And it also prevents someone from just loosing up the buckles and taking the whole box out.
I treated it with Thompson's Water Seal, Only bad thing is they wanted to Cure for 45 days, before you paint it.
After I let it cure, I used bedliner, So it would not scratch up. And the exterior would be more durable when I start throwing crap on top of it! The bedliner is not real rough but it does provide antiskid properties. I also wanted it blacked out so if somebody looked inside they could not see it. It locks up and the only way you can get to the lock is if you open the tailgate.
I went to salvage yard and got the hood Rod off an old CJ To hold the cover of the box up when I need to get into it. I mounted a battery-operated LED to the cover inside So I can see what I'm looking at inside the box.
Around the inside of the box I mounted these angle Brackets that have holes and slots in them. I can zip tie stuff to those brackets that I don't want sliding around, Or things that I need and want quick access too. I also have a thin piece of plywood that fits on top of that bracket that acts like a false floor.