The theory still the same, but takes a bit more time. Do you have a
A coping saw
A large enough flat enough surface to work on (where a whole sheet of sand paper can fit) for the curved part get one of those sanding blocks that are a solid piece of rubber.
I do have the knowledge how to thin scale material without power tools (check my gallery for how they turn out), but I was hoping you would share some tips for the OP on how to if you would not mind.