This are some little sanding sticks I made to get access to some very difficult to reach areas, which very frequently occur doing luthiery.
You need them in various sizes and various radii.
I did them using either PVC strips, shaped with the heat gun, or steel sheet, put un shape with the hammer on a big tube.
The sand paper is hold with double face tape.
They can be used on both sides, convex and concave.