I’m often annoyed at myself for failing to make holes perpendicular to the surface of my workpiece, when using a hand-drill.
The world is full of gadgets which aim to help, all of which seem pretty complex. Here is my alternative (which just sounds complex ; )
Think of a ring of metal which has three struts pin-jointed onto it. Each strut is thus free to move so as to stand at 90 degrees to the plane of the ring or to lie flat within that plane. The three struts, when lying in the plane, form an equilateral triangle. A similar set of joints exists between the other ends of the struts and another ring. This ensures that the two rings stay in parallel planes, even when the struts fold (a bit like two picnic tables joined leg-end to leg-end).
This arrangement would be sprung so as to maximally separate the two rings. All this geometry would be attached to the body of a hand-drill, with the drill bit concentric to the two rings.
When you want to drill a perpendicular hole in a surface (even if it’s spherical) you place the lower ring on the surface and move the bit in close to the drill mark. This compresses the spring, and the whole device, purely axially as you drill into the surface. At all times you can see where the bit is going and if the lower ring is always in contact with the workpiece, you get the right result. This guide avoids the need to have a range of different sized inserts, one for each drill bit diameter.
If you need a longer axial travel than this allows, these units can be concatenated to provide it. Although I’m generally dead against making things in plastic when they really need the rigidity/toughness of metal, it might even be possible to implement this design as a one-piece injection moulding, so that the springiness could be a feature of the moulded joints themselves.