I cut the hole for the light, then drilled for the ribs before final riveting of the skin. Removing the skin for deburring and final cleanup worked well for me. Also gave me lots of room for getting everything fitted nicely. No doubt there are success stories for all methods, but this was what I did.
Have fun. Cutting that hole was a little scary at first, but it isn't that hard. A unibit for the corners, and a Dremel cut off wheel for the straight lines, then finished with a sanding drum. A Dremel with a flex shaft is truly a miracle tool!
All the best,
DaveH
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