Quick hardware question: tool box to cargo carrier

Thanks to everyone for the advice, I used bits and pieces of everyone's input it seems like. here are the pics, the attachments inside are two small clevis pins with fender washers underneath in the factory holes (more for stability than anything else) and a square u-bracket right in the center over the main carrier frame element for strength and to actually anchor the box to the cargo rack. I think this will work out pretty well.