I would also recommend grade eight hardware to bolt the box to the frame. Common Chinese; or made any where else off shore hardware bolts from Home Depot or Lowes are far to weak to retain the box once you load it up (barely qualify as a SAE Grade 2). They are distinguished by a flat head with no marking on them. Grade five which is standard automotive duty has three slashes on the head; SAE grade eight will have six slashes on the head if English. If metric it will have a number stamped on the head that isn't the same as a SAE grade eight; as an 8.8 isn't a SAE Grade 8.8 but the metric equivalent of a SAE Grade 5. The SAE grade 8 will be marked 10.9.
Finally as if this isn't confusing enough the ASTM (a testing society for building components) has their own rating of bolts so you could find a ASTM rated bolt marked A325 for an equivalent SAE (Society of Automotive Engineers) of a grade 8. The grade 5 would be marked as A307. Most bolts in buildings and bridges are pretty hefty and have almost exclusively a higher rating than a grade 8; as clamping force is paramount (most buildings don't move much so there isn't a lot of thought given to bending moments). But you could find these bolts at a building site if you worked there and forgot to take some out of your pocket when you went home.
Excessively hard isn't a good thing in automobiles that flex and bend as they follow the road or attempt to turn. You need a high tensile strength without excessive hardness which equates to being brittle. A case hardened bolt has a higher tensile strength and can resist shear better than softer bolt but it could also fail from being bent repeatedly. Instead you need a stronger; not a harder bolt (which doesn't play well into a stainless steel bolts strengths). Instead you need alloys of silicon and manganese with chrome and carbon. You need an ARP bolt.