I usually use a surface mount wall pack that doesn't need a junction box. However, that last metal sided building, the owners wanted goose neck lighting and I had to use a box. This is dependent on the inside not being finished...yet. I ran a vertical 2X6 between two of the horizontal boards, centered on the rib where the lights were to go. I ran 3/4" plywood from the top of the top horizontal board to the bottom of the bottom horizontal and over my vertical 2X6 and screwed it all together. Then I ran a 4" holesaw through the metal and all the way through the 2X6, but only let the pilot bit go into the plywood, that way, the center of the hole is marked. Then I changed holesaws to one that was just big enough for the connector (mc cable in my case) and using the pilot bit hole in the plywood, ran that holesaw through the plywood. Then it was easy, ran mc cable through the hole, into the back of an octagon box, then screwed the box to the plywood. It was kind of a pain in the ****, but turned out nice.
You could also surface mount a weatherproof light box to the metal, but make sure you put it where you screw into the framing. Another one, but I haven't tried this so I'm not sure how well it would work. Using a 4" holesaw, go through the metal and 3/4" into a framing member and chip out the wood to create a 4" X 3/4" deep recess and install a pancake box.