Of course, you could also use the bandura soundboard as an
excuse to buy another tool! I'd go for one of those work-of-art
Laarman violin makers' planes.  ;->

What Ralph is proposing sounds a lot more sensible than my
approach (using a block plane). Although I typically plane a
piece hollow with a light set using the aforementioned technique
(to make sure that I'm not planing it convex) I don't typically
work with thin stock. Ralph, as the Porch's Shaker box expert,
does a lot of this sort of work.

Regards,

Kelly Cox
Madison, WI