You get your wish
Hope this helps fellow builders in the future.
John-G (and Curt and Scott): It definitely did help, thanks ever so much.
I mounted the landing light yesterday. Like everyone says, it?s kinda daunting, but a thoughtful and careful approach carried the day.
Having read all the above, I determined to start out with a known accurate template. I laid out each part onto the template sheet and verified all the hole locations and spacings, including the rib rivet locations used as the reference datum. All OK.
Then I took the template to Staples and had them run three copies onto a lightweight card stock, checking and double-checking to make sure they came out the exact same size. They did.
Rechecked all locations and spacings on the new template. Still good. I then put the template on a smooth benchtop and pricked each hole location ?x? mark dead center with a sharp pointed alignment punch. Used the same punch to open up the three reference holes on the left edge for the existing clecos (photo).
Placing the template, I put down a few strips of double sided tape to help keep it from moving; also added a couple pieces of masking tape on the edges as needed.
An automatic center punch adjusted to a light setting nicely self-located in the tiny pre-punched hole at each ?x? mark. I used a small block of wood on the inside to back it up.
Good results all around. I have no idea if I could have gotten the same happy ending (photo) had I not first read this thread.
Thanks again, guys! Now I?m hoping my addition might pay it forward to the next builder.