I've always used a thin piece of aluminum about 18" long as a template with pre-drilled holes spaced at 1.5". I offset the holes so that they are closer to one edge than the other (~1/2") so it's easier to align along an edge. Once you hold/clamp it in place you can drill your first pilot hole and cleko it in place or use one of the FFR predrilled holes for proper alignment. The thin sheet metal template allows you to conform to curves/right angles as needed.
This is the spacer from AirCraft Spruce - well worth the $36. You don't want to drill through it - you use it as a layout tool. I did drill through the hole at one end, then I would Cleco it to the panel and stretch it out to get the proper rivet spacing and make my marks where to drill
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