Sometimes I find that if you don't have a tool to do the job you can do well by making your own. I was having trouble sanding the interior concave curve of the upper bout. With the aide of stick on sandpaper that you buy in big rolls, I made this little sander and find it works wonderfully,
The secret here is the sandpaper that has adhesive on the back. you just wrap it around the piece of wood and you have a curved sander on one side and a flat sander on the other- I Love it. Really works well.