If it's anything like my 9, you will be drilling and dimpling those holes before the trim tab is closed up for good. You can shape the ends, cleco everything in place and drill, and then disassemble, open the tab up enough to put your dimpler in there, and go to town. You could use a pop-rivet dimpler as well, but I was able to slide the squeezer in no problem. Just finished my 2nd one (first one was ruined trying to rivet the horns on when they weren't totally flush; drilling out the rivets cracked the skin). When you go to attach the horns, remember that dimpling them will cause the flat surface to be distorted. Carefully work it back flat again so that it sit's perfectly flush before riveting. Also bend the tabs before attaching the horns. At least in the -9 instructions, it has you attach horns first, which makes riveting them easier, but bending the tabs almost impossible.
Chris