If the neck is already finished and you can't sand it down and you have hand tools only? OK...Take a small block of metal (or an old tremblock) that has nice square sides (to keep the pocket sidewalls perpendicular to the floor). Apply adhesive type sandpaper or glue sandpaper to only the sides of the metal block and sand the neck pocket's sidewalls and/or rear wall until the neck fits. Don't make the fit too tight to allow for expansion with humidity and fretboard string alignment.