Quick hardware question: tool box to cargo carrier

First make sure your truck has enough engine cooling as this will lower radiator efficiency.
Easy way to drill 4 holes in the bottom and put bolts with nuts & counter-nuts inside. This way nobody can unbolt it from the outside.
I also like to use metric or even torx head bolts to give potential thief more headaches.
I carry 130 lb generator on my front hitch and fabricated dual-sided rubber mounts to lower vibration.