Here's the final (for now) version of the rivet spacing tool I have been working on. This one is smaller than the first try, which makes it less floppy. I only need a maximum of 3" spacing, which this provides nicely. And, for the aesthetics of the thing, I modified a couple of the legs to remove an unneeded sticky-outy bit and an excess hole.
Now I can get back to marking and mounting the next panel.