I don't use boxes for wallpacks anymore. I use MC cable in a situation like yours (metal siding). If the fixture has no 1/2" knockouts(the ones I have seen have at least one) I make my own, and I have used duplex MC connectors for two cables or sometimes made my own second KO. I try to duct seal the hole or silicone it before I push the fixture against the wall to waterproof it. Getting it to sit evenly on deeply ribbed metal siding IS a pain, and I too would like to see some input here, as mounting them securely is very difficult on a steel building with beams and purlins and no wood backing.
