Maybe I am just slow.
Confronted with the need to drill some grab iron holes in an awkward place (the side of a UTLX tank car running board--sorry, I am being distracted), I thought about a means to accurately locate the spot without clamping the part or needing five hands. Then It came to me:
Tape!
I measured the tape, wrapped it around the running board and got some darn consistent holes.
Today the job is to fabricate grab irons with a curve to them. Again, tape comes to the rescue.

It works great on flat and curved surfaces.

Drilling holes and making grab irons is toward the bottom of the list of favorite modeling tasks, and this makes the results more reliable and thus more pleasurable.
Try it yourself.
Keith Hayes
Leadville in Sn3