You can easily make a selection of concave sanding tools out of short lengths of PVC (or other material) pipe.
I've made a number of them using pipe of different diameters (PVC works best and is easy to cut).
Simply cut to desired length...you might want some long (like for leading edges), or short for utility work.
Next cut in half, quarters, or even thirds lengthwise..
Use sticky-back sandpaper of the grit appropriate for the task.
Bonus: If you wanted to sand a concave surface on the plane then just put the sandpaper on the outside of the piece of pipe.
Cheers,
G42