If what you're after is what i'm thinking, you can get a rubber sanding block from b&q and while there is a flat face on one side, the other side is rounded. It's designed that way so you can use it comfortably in your hand but you can flip it and use it as a concave sander.
I use this on my bats and because it is rubber it will flex to fit the contours of the bat. This is what I mean:
You can see the slots in the rubber at each end, there are metal pins that stick out that you use to grip the sandpaper (you will need to cut it down to fit).
Hope this helps.