theres lots of options for ataching it to the box.
you could just screw it and unscrew it everytime you use it. the one turtleman and i made uses ropes that tie to the box so we just untie when we put it away
dude, instead of 2x6s, use two 8' studs and a 2x8' sheet of ply wood. you can toss it on the roof if you need to but most suvs can have a sheet of ply inside 'em