Quick hardware question: tool box to cargo carrier

4 bolts thru the box and the support frame, not the mesh. Put a rubber washer under the bolt head if you’re wanting to keep it afloat.
Or little u bolts if you don’t want to drill the frame on the rack.

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.