Often you find yourself in a position where you need to drill a hole for a fastener located underneath a structure, but there is no way you can locatie that hole with a tool like a hole doubler/copier.
In that case, the preferred way is to use little magnetic balls that attract and pin point the right location to drill the hole.
That sounds better than it actually is when you try it in real life.
The magnets indeed attract eachother but the exact location is hard to find. Slightly moving the locator magnet may be quite a bit off the actual center depending on how you push or move it.
I encountered this problem the first time on the canopy drilling and later also on the drilling of holes on the wheel pants.
I recently found this video below from DanM who explains a much more precise method on how to pin point the exact location of the drill hole when installing the skybolt receptacles on the cowling.
This method is applicable everywhere the nutplate or fastener location underneath is unreachable.
See the link below for the full video showing how to use a smalll needle on a string to locate the exact drilling location. It even works sideways or upside down.
Fantastic tip !
Here is the youtube video link.