I can pretty much garuntee, any bodyman/bodyshop will tell you just to replace the bumper. That it isnt worth the effort and time to try to fix it. Because you will never get it straight. At the bodyshop I used to work at. anything with a chrome bumper that even had a slight dent. Was tossed out for a new one.

And personally, I think a toolbox for a rear bumper is an invitation for theives. So probably not the best idea, unless you like having your crap stolen...Take it as you will though.