I think it'll be very hard to do with an exacto knife. You might go at it with a hack saw blade. Wrap both ends of the blade with tape and work slowly in from to the dihedral brace, then from the back to the dihedral brace. The tough part will be the brace itself. You may need to cut through it and make a new one out of some ply. Use the sheet the original brace came from as a guide to mark and cut a new brace.

For the sanding block, use as long a block as you can. The longer the block, the more even and true the surfaces will be.

Scott