Just a thought, but if there is enough room in front of the box, you could attach a hitch receiver under the tool box and use one of the bike racks you see on all the SUV's. Something like this.
http://www.rvtoyoutlet.com/c-listall/p-RV8170.html
That would also make it removable if not needed. The only perm attached part is well out of the way.